NameError: name 'gc' is not defined

**Problem Description**
NameError: name 'gc' is not defined in chaco/chaco/plots/polar_line_renderer.py

**Reproduction Steps:**
python chaco/examples/demo/simple_polar.py

[Steps to reproduce issue here.]

```python
# Code to reproduce issue here
```

**Expected behavior:**  

[MacOS, python3.8 (EDM)]

**OS, Python version:** [Enter OS name and Python version]
