Heatmap Padding

[CalvinFernandez]

Add xgap and ygap field to heat map that allows users to define space between bricks.

closes #868

[etpinard]

@CalvinFernandez Thanks so much for this PR!

Looks like mocks hist2d_summed, 22 and 21 fail to render the heatmap.

Also, looks like jasmine test heatmap plot draws canvas with correct margins yields some cross-browser differences (we use FF on CI). You can try running that suite locally on FF with

npm run citest-jasmine -- tests/heatmap_test.js

If you have any questions about the two points above ⬆️ please let me know.

[etpinard]

@CalvinFernandez tests are passing, this PR is looking great.

I just thought about one little detail that should be fixed before merging. Right now, xgap and ygap only have an effect for brick-like heatmaps and 2D histograms (i.e. with zsmooth set or infer as false). So we should make sure that xgap and ygap are coerced only when zsmooth allows them to have an effect. In other words, xgap and ygap are unsettable when zsmooth is set to 'best' or 'fast'. Unsettable attributes are caught by the Plotly.validate method allowing users to catch bad inputs.

For example:

var data = [{ y: [2,1,2], mode: 'markers', line: { shape: 'spline' } }];

Plotly.validate(data);
// => In data trace 0, container line did not get coerced

To do so, corce zsmooth before xgap and ygap in the heatmap and histogram2d defaults as such:

var zsmooth = coerce('zsmooth');

if(zsmooth === false) {
  coerce('xgap');
  coerce('ygap');
}

Thanks again for your efforts 🍻

[etpinard]

Amazing PR. Thank you very much 🍻