diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 3c68e212..399c1330 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -25,6 +25,9 @@ Unreleased
    in them (@nkorinek, #182)
 -  added contributors file and updated README to remove that information
    (@nkorinek, #121)
+-  Made checking line coverage optional for `base.assert_line()`
+   (@ryla5068, #239)
+-  Fixed bugs involving line tests (@ryla5068, #239)
 
 0.1.2
 -----
diff --git a/dev-requirements.txt b/dev-requirements.txt
index 18de722e..8c89ea2e 100644
--- a/dev-requirements.txt
+++ b/dev-requirements.txt
@@ -10,4 +10,5 @@ setuptools==46.1.3
 pre-commit==1.20.0
 pip==19.0.3
 descartes==1.1.0
+seaborn>=0.9.1
 pillow==7.1.2
diff --git a/matplotcheck/base.py b/matplotcheck/base.py
index bef8cec2..04dc3052 100644
--- a/matplotcheck/base.py
+++ b/matplotcheck/base.py
@@ -1061,13 +1061,12 @@ def assert_line(
         self,
         slope_exp,
         intercept_exp,
-        xtime=False,
+        check_coverage=False,
         message_no_line="Expected line not displayed",
         message_data="Line does not cover data set",
     ):
         """Asserts that there exists a line on Axes `ax` with slope `slope_exp`
-        and y-intercept `intercept_exp` and goes at least from x coordinate
-        `min_val` to x coordinate `max_val`
+        and y-intercept `intercept_exp` and
 
         Parameters
         ----------
@@ -1075,46 +1074,60 @@ def assert_line(
             Expected slope of line
         intercept_exp : float
             Expeted y intercept of line
-        xtime : boolean
-            Set ``True`` if x-axis values are datetime
+        check_coverage : boolean
+            If `check_coverage` is `True`, function will check that the goes at
+            least from x coordinate `min_val` to x coordinate `max_val`. If the
+            line does not cover the entire dataset, and `AssertionError` with
+            be thrown with message `message_data`.
         message_no_line : string
             The error message to be displayed if the line does not exist.
         message_data : string
             The error message to be displayed if the line exists but does not
-            cover the dataset.
+            cover the dataset, and if `check_coverage` is `True`.
 
         Raises
         -------
         AssertionError
-            with message `m` or `m2` if no line exists that covers the dataset
+            with message `message_no_line` or `message_data` if no line exists
+            that covers the dataset.
         """
-        flag_exist, flag_length = False, False
-        xy = self.get_xy(points_only=True)
-        min_val, max_val = min(xy["x"]), max(xy["x"])
+        flag_exist = False
+
+        if check_coverage:
+            flag_length = not check_coverage
+            xy = self.get_xy(points_only=True)
+            min_val, max_val = min(xy["x"]), max(xy["x"])
 
         for l in self.ax.lines:
-            path_verts = self.ax.transData.inverted().transform(
-                l._transformed_path.get_fully_transformed_path().vertices
-            )
+            # Here we will get the verticies for the line and reformat them in
+            # the way that get_slope_yintercept() expects
+            data = l.get_data()
+            path_verts = np.column_stack((data[0], data[1]))
+
             slope, y_intercept = self.get_slope_yintercept(path_verts)
             if math.isclose(slope, slope_exp, abs_tol=1e-4) and math.isclose(
                 y_intercept, intercept_exp, abs_tol=1e-4
             ):
                 flag_exist = True
-                line_x_vals = [coord[0] for coord in path_verts]
-                if min(line_x_vals) <= min_val and max(line_x_vals) >= max_val:
-                    flag_length = True
-                    break
+                if check_coverage:
+                    line_x_vals = [coord[0] for coord in path_verts]
+                    if (
+                        min(line_x_vals) <= min_val
+                        and max(line_x_vals) >= max_val
+                    ):
+                        flag_length = True
+                        break
 
         assert flag_exist, message_no_line
-        assert flag_length, message_data
+        if check_coverage:
+            assert flag_length, message_data
 
     def assert_lines_of_type(self, line_types):
         """Asserts each line of type in `line_types` exist on `ax`
 
         Parameters
         ----------
-        line_types : list of strings
+        line_types : string or list of strings
             Acceptable strings in line_types are as follows
             ``['regression', 'onetoone']``.
 
@@ -1127,32 +1140,40 @@ def assert_lines_of_type(self, line_types):
         -----
             If `line_types` is empty, assertion is passed.
         """
-        if line_types:
-            for line_type in line_types:
-                if line_type == "regression":
-                    xy = self.get_xy(points_only=True)
-                    slope_exp, intercept_exp, _, _, _ = stats.linregress(
-                        xy.x, xy.y
-                    )
-                elif line_type == "onetoone":
-                    slope_exp, intercept_exp = 1, 0
-                else:
-                    raise ValueError(
-                        "each string in line_types must be from the following "
-                        + '["regression","onetoone"]'
+        if isinstance(line_types, str):
+            line_types = [line_types]
+
+        for line_type in line_types:
+            if line_type == "regression":
+                xy = self.get_xy(points_only=True)
+                # Check that there is xy data for this line. Some one-to-one
+                # lines do not produce xy data.
+                if xy.empty:
+                    raise AssertionError(
+                        "regression line not displayed properly"
                     )
-
-                self.assert_line(
-                    slope_exp,
-                    intercept_exp,
-                    message_no_line="{0} line not displayed properly".format(
-                        line_type
-                    ),
-                    message_data="{0} line does not cover dataset".format(
-                        line_type
-                    ),
+                slope_exp, intercept_exp, _, _, _ = stats.linregress(
+                    xy.x, xy.y
+                )
+            elif line_type == "onetoone":
+                slope_exp, intercept_exp = 1, 0
+            else:
+                raise ValueError(
+                    "each string in line_types must be from the following "
+                    + '["regression","onetoone"]'
                 )
 
+            self.assert_line(
+                slope_exp,
+                intercept_exp,
+                message_no_line="{0} line not displayed properly".format(
+                    line_type
+                ),
+                message_data="{0} line does not cover dataset".format(
+                    line_type
+                ),
+            )
+
     # HISTOGRAM FUNCTIONS
 
     def get_num_bins(self):
diff --git a/matplotcheck/tests/test_base_lines.py b/matplotcheck/tests/test_base_lines.py
new file mode 100644
index 00000000..9318d9e9
--- /dev/null
+++ b/matplotcheck/tests/test_base_lines.py
@@ -0,0 +1,117 @@
+import pytest
+from matplotcheck.base import PlotTester
+import matplotlib.pyplot as plt
+import pandas as pd
+import seaborn as sns
+from scipy import stats
+
+"""Fixtures"""
+
+
+@pytest.fixture
+def pd_df_reg_data():
+    """Create a pandas dataframe with points that are roughly along the same
+    line."""
+    data = {
+        "A": [1.2, 1.9, 3.0, 4.1, 4.6, 6.0, 6.9, 8.4, 9.0],
+        "B": [2.4, 3.9, 6.1, 7.8, 9.0, 11.5, 15.0, 16.2, 18.6],
+    }
+
+    return pd.DataFrame(data)
+
+
+@pytest.fixture
+def pt_reg_data(pd_df_reg_data):
+    """Create a PlotTester object with a regression line"""
+    fig, ax = plt.subplots()
+    sns.regplot("A", "B", data=pd_df_reg_data, ax=ax)
+
+    return PlotTester(ax)
+
+
+@pytest.fixture
+def pt_one2one():
+    """Create a PlotTester object a one-to-one line"""
+    fig, ax = plt.subplots()
+    ax.plot((0, 1), (0, 1), transform=ax.transAxes, ls="--", c="k")
+
+    return PlotTester(ax)
+
+
+@pytest.fixture
+def pt_reg_one2one(pd_df_reg_data):
+    """Create a PlotTester object with a regression line and a one-to-one
+    line"""
+    fig, ax = plt.subplots()
+    sns.regplot("A", "B", data=pd_df_reg_data, ax=ax)
+    ax.plot((0, 1), (0, 1), transform=ax.transAxes, ls="--", c="k")
+
+    return PlotTester(ax)
+
+
+def test_reg_plot(pd_df_reg_data, pt_reg_data):
+    """Test that assert_line() correctly passes when given the correct slope
+    and intercept."""
+    # Get the correct slope and intercept for the data
+    slope_exp, intercept_exp, _, _, _ = stats.linregress(
+        pd_df_reg_data.A, pd_df_reg_data.B
+    )
+
+    pt_reg_data.assert_line(slope_exp, intercept_exp)
+
+
+def test_reg_plot_slope_fails(pd_df_reg_data, pt_reg_data):
+    """Check that assert_line() correctly falis when given an incorrect
+    slope."""
+    _, intercept_exp, _, _, _ = stats.linregress(
+        pd_df_reg_data.A, pd_df_reg_data.B
+    )
+    with pytest.raises(AssertionError, match="Expected line not displayed"):
+        pt_reg_data.assert_line(1, intercept_exp)
+
+
+def test_reg_plot_intercept_fails(pd_df_reg_data, pt_reg_data):
+    """Check that assert_line() correctly fails when given an incorrect
+    intercept"""
+    slope_exp, _, _, _, _ = stats.linregress(
+        pd_df_reg_data.A, pd_df_reg_data.B
+    )
+
+    with pytest.raises(AssertionError, match="Expected line not displayed"):
+        pt_reg_data.assert_line(slope_exp, 1)
+
+
+def test_line_type_reg(pt_reg_data):
+    """Check that assert_lines_of_type() correctly passes when checking for a
+    regression line."""
+    pt_reg_data.assert_lines_of_type("regression")
+
+
+def test_line_type_one2one(pt_one2one):
+    """Check that assert_lines_of_type() correctly passes when checking for a
+    one-to-one line."""
+    pt_one2one.assert_lines_of_type("onetoone")
+
+
+def test_line_type_reg_one2one(pt_reg_one2one):
+    """Check that assert_lines_of_type() correctly passes when checking for
+    both a regression line and a one-to-one line."""
+    pt_reg_one2one.assert_lines_of_type(["regression", "onetoone"])
+
+
+def test_line_type_reg_fails(pt_one2one):
+    """Check that assert_lines_of_type() correctly fails when checking for a
+    regression line, but one does not exist."""
+    with pytest.raises(
+        AssertionError, match="regression line not displayed properly"
+    ):
+        pt_one2one.assert_lines_of_type("regression")
+
+
+def test_line_type_one2one_fails(pt_reg_data):
+    """Check that assert_lines_of_type() correctly fails when checking for a
+    one-to-one line, but one does not exist."""
+    with pytest.raises(
+        AssertionError, match="onetoone line not displayed properly"
+    ):
+        pt_reg_data.assert_lines_of_type("onetoone")