Don't mutate colorscale, cmin/cmax and zmin/zmax into user traces

[etpinard]

This PR fixes two bugs (#3273 and #3100) using a similar technique to #3044.

In brief, we use Colorscale.crossTraceDefaults to put _ attributes (relinked in relinkPrivateKeys) into fullData - instead of mutating values into gd.data.

I also cleaned up a few things in components/colorscale/:

• merge helpers into two files: scales.js and helpers.js
• apply reversescale later in the pipeline (just before plotting) instead of mutating the colorscale value in gd._fullData (saving us some headaches in crossTraceDefaults).

asking @plotly/plotly_js for a review 😏

@nicolaskruchten would mind testing out 6a47d66 in the RCE? Thank you!

[nicolaskruchten]

Oh wow that works great :)

[archmoj]

Great PR! It may likely hit the record of the number of source files changed in one PR in 2018?
@etpinard After a quick first pass & concerning the change in this baseline; I was wondering which version is correct, old or new? The background color in the hover popup does not display the new color.

[etpinard]

After a quick first pass & concerning the change in this baseline; I was wondering which version is correct, old or new?

Thanks very much @archmoj ! It turns out both versions were wrong! The version from commit 0a686dd looked worse than the one on master, as now we no longer flip gd._fullData[i].colorscale when reversescale is true, but it was off nonetheless. See fix in 175626b as well as other corrected baselines in fc884a7

[nicolaskruchten]

Out of curiosity, what's the new logic for the legend marker?

[etpinard]

Out of curiosity, what's the new logic for the legend marker?

There's no new logic really. This PR fixed Lib.minExtend for colorscale values. Lib.minExtend is used in the legend-styling code to mock calcdata so that we can reuse our cartesian Drawing methods to draw legend items. Commit 0a686dd made this bug more obvious, but things were broken before.

[nicolaskruchten]

OK, but what's the one-liner english description of the logic? We grab a marker from the middle of the scale or... ?

[etpinard]

OK, but what's the one-liner english description of the logic?

We grab the first marker.color item

plotly.js/src/components/legend/style.js

Line 183 in 431fd51

dEdit.mc=boundVal('marker.color',pickFirst);

and then apply the marker.colorscale if present.

[archmoj]

Many thanks @etpinard
Big improvements indeed!
💃 from my side.

[etpinard]

Thanks very much for the thorough review!