Improve CONTRIBUTING.md

CONTRIBUTING.md

@@ -17,6 +17,7 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 | `type: bug` | bug report confirmed by a plotly team member |
 | `type: regression` | bug that introduced a change in behavior from one version to the next |
 | `type: feature` | planned feature additions |
+| `type: new trace type` | subset of `type: feature` reserved for planned new trace types |
 | `type: translation` | localization-related tasks |
 | `type: performance` | performance related tasks |
 | `type: maintenance` | source code cleanup resulting in no enhancement for users |
@@ -96,6 +97,13 @@ Three additional helpers exist that are refreshed every second:
 There is also a search bar in the top right of the dashboard. This fuzzy-searches
 image mocks based on their file name and trace type.
 
+#### Alternative to test dashboard
+
+Use the [`plotly-mock-viewer`](https://github.com/rreusser/plotly-mock-viewer)
+which has live-reloading and a bunch of other cool features.
+An online version of `plotly-mock-viewer` is available at https://rreusser.github.io/plotly-mock-viewer/
+which uses https://cdn.plot.ly/plotly-latest.min.js
+
 #### Other npm scripts
 
 - `npm run preprocess`: pre-processes the css and svg source file in js. This
@@ -175,11 +183,23 @@ which shows the baseline image, the generated image, the diff and the json mocks
 
 To view the results of a run on CircleCI, download the `build/test_images/` and `build/test_images_diff/` artifacts into your local repo and then run `npm run start-image_viewer`.
 
-### Writing interaction tests
+### Using the developer console in karma to write/debug jasmine tests
+
+- Click on the `DEBUG` button
+- In the `DEBUG RUNNER` window, open the console (e.g. with `<ctrl-shift-j>`)
+- Find test file (e.g. with `<ctrl-o>` + typing the name of the file), look out
+  for "bundled" files with the same name.
+- Set `debugger` on relevant line(s)
+- Rerun the test suite by refreshing the page (e.g. with `<crtl-r>`)
+
+![Peek 2020-03-11 10-45](https://user-images.githubusercontent.com/6675409/76438118-f2502300-6390-11ea-88d2-17a553c3b4e8.gif)
+
+### Writing jasmine interaction tests
+
 Keep in mind that the interaction coordinates are relative to the top-left corner of the plot, including the margins. To produce a reliable interaction test, 
 it may be necessary to fix the width, height, margins, X axis range and Y axis range of the plot. For example:
 
-```
+```js
 Plotly.newPlot(gd, [{
     x: [1, 1, 1, 2, 2, 2, 3, 3, 3],
     y: [1, 2, 3, 1, 2, 3, 1, 2, 3],
@@ -196,18 +216,66 @@ This will produce the following plot, and say you want to simulate a selection p
 
 <img src="https://user-images.githubusercontent.com/31989842/38890553-0bc6190c-4282-11e8-8efc-077bf05ca565.png">
 
-
 ## Repo organization
 
 - Distributed files are in `dist/`
 - CommonJS require-able modules are in `lib/`
-- Sources files are in `src/`, including the index
+- Sources files are in `src/`
 - Build and repo management scripts are in `tasks/`
 - All tasks can be run using [`npm run-script`](https://docs.npmjs.com/cli/run-script)
 - Tests are `test/`, they are partitioned into `image` and `jasmine` tests
 - Test dashboard and image viewer code is in `devtools/`
-- Built files are in `build/` (most files in here are git-ignored, the css and font built files are exceptions)
-
+- Built files are in `build/` (the files in here are git-ignored, except for `plotcss.js`)
+
+## Trace module design
+
+The trace modules (found in [`src/traces`](https://github.com/plotly/plotly.js/tree/master/src/traces))
+are defined as plain objects with functions and constants attached to them in an index file
+(e.g. `src/traces/scatter/index.js`). The trace modules are "registered" undo the `Registry` object
+(found in [`src/registry.js`](https://github.com/plotly/plotly.js/blob/master/src/registry.js)) using
+`Plotly.register` (as done in the index files in `dist/`).
+
+The trace module methods are meant to be called as part of loops during subplot-specific
+(e.g. in `plots/cartesian/index.js`) and figure-wide (e.g. in `plots/plots.js`) subroutines.
+That way, the subroutines work no matter which trace modules got registered.
+
+All traces modules set:
+
+- `_module.name`: name of the trace module as used by the trace `type` attribute.
+- `_module.basePlotModule`: base plot (or subplot) module corresponding to the
+  trace type (e.g. `scatter` links to the `Cartesian` base plot module, `scatter3d` links to `gl3d`).
+- `_module.attributes`: JSON-serializable object of attribute declarations.
+  This object is used to generate the plot-schema JSON.
+- `_module.supplyDefaults`: Takes in input trace settings and coerces them into "full" settings
+  under `gd._fullData`. This one is called during the figure-wide `Plots.supplyDefaults` routine.
+  Note that the `supplyDefaults` method performance should scale with the number of attributes (**not** the
+  number of data points - so it should not loop over any data arrays).
+- `_module.calc`: Converts inputs data into "calculated" (or sanitized) data. This one is called during
+  the figure-wide `Plots.doCalcdata` routine. The `calc` method is allowed to
+  scale with the number of data points and is in general more costly than `supplyDefaults`.
+  Please note that some edit pathways skip `Plots.doCalcdata` (as determined by the
+  `editType` flags in the attributes files).
+- `_module.plot`: Draws the trace on screen. This one is called by the defined `basePlotModule`.
+
+Other methods used by some trace modules:
+
+- `_module.categories`: list of string identifiers used to group traces by behavior. Traces that
+   have a given category can then be detected using [`Registry.traceIs`](https://github.com/plotly/plotly.js/blob/8f049fddbac0ca0382816984b8526857e9714fe6/src/registry.js#L129-L155)
+- `_module.layoutAttributes`: JSON-serializable object of attribute declarations
+  coerced in the layout (e.g. `barmode` for `bar` traces)
+- `_module.supplyLayoutDefaults`: Defaults logic for layout attributes.
+- `_module.crossTraceDefaults`: Defaults logic that depends on input setting of multiple traces.
+- `_module.crossTraceCalc`: Computations that depend on the data of multiple traces.
+- `_module.colorbar`: Defines the colorbar appearance for traces that support it.
+- `_module.hoverPoints`: Point-picking logic called during hover.
+- `_module.selectPoints`: Polygon-containing logic called during selections.
+- `_module.style`: Sometimes split from `_module.plot` where `_module.plot` only
+  draws the elements and `_module.style` styles them.
+- `_module.styleOnSelect`: Optimization of `_module.style` called during
+  selections.
+- `_module.convert`: Sometimes separated from `_module.plot` or `_module.calc` to convert the
+  plotly.js settings to another framework e.g. to `gl-plot3d` for `gl3d` traces, to
+  `mapbox-gl` from `mapbox` traces. This split can make the logic easier to test.
 
 ## Coding style
 

test/image/README.md

@@ -93,8 +93,11 @@ For example,
 # Run one test (e.g. the 'contour_nolines' test):
 $ npm run test-image -- contour_nolines
 
-# Run all gl3d image test in batch:
+# Run all gl3d image tests in batch:
 $ npm run test-image -- gl3d_*
+
+# Run all image tests that are not gl3d in batch:
+npm run test-image --  "\!\(gl3d_\)*"
 ```
 
 Developers on weak hardware might encounter batch timeout issue. These are most