Fix boxplot with categoryorder

[antoinerg]

Closes #3970

This seems a bit hacky to me to it does fix the issue at hand.

[antoinerg]

Codepens:

• Before: https://codepen.io/anon/pen/LKZXRy
• After: https://codepen.io/antoinerg/pen/rEzGoL

[etpinard]

Thanks for the fix. Nicely done!

While at it, it would be nice to investigate if the other "per-graph" variables initialised in Plots.doCalcdata:

plotly.js/src/plots/plots.js

Lines 2755 to 2764 in 795220f

	fullLayout._violinScaleGroupStats = {};
	// for calculating avg luminosity of heatmaps
	gd._hmpixcount = 0;
	gd._hmlumcount = 0;
	// for sharing colors across pies / sunbursts / funnelarea (and for legend)
	fullLayout._piecolormap = {};
	fullLayout._sunburstcolormap = {};
	fullLayout._funnelareacolormap = {};

lead to similar bugs when sorting categories per value.

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Regardless this is a great fix, so 💃