Show box/violin position when hovering over pts

[etpinard]

... instead of showing the point's position - fixes #3440

https://codepen.io/etpinard/pen/NeVaYR?editors=1010

now looks like:

Note that this was only a problem for box/violin traces with numeric positions (e.g. with set x and y arrays) under hovermode:'closest', as deeper down Fx.hover

plotly.js/src/components/fx/hover.js

Lines 1341 to 1348 in cc45972

	if(d.xLabelVal !== undefined) {
	d.xLabel = ('xLabel' in d) ? d.xLabel : Axes.hoverLabelText(d.xa, d.xLabelVal);
	d.xVal = d.xa.c2d(d.xLabelVal);
	}
	if(d.yLabelVal !== undefined) {
	d.yLabel = ('yLabel' in d) ? d.yLabel : Axes.hoverLabelText(d.ya, d.yLabelVal);
	d.yVal = d.ya.c2d(d.yLabelVal);
	}

calls Axes.hoverLabelText which then formats the positions back to categories with:

plotly.js/src/plots/cartesian/axes.js

Lines 1164 to 1168 in cc45972

	function formatCategory(ax, out) {
	var tt = ax._categories[Math.round(out.x)];
	if(tt === undefined) tt = '';
	out.text = String(tt);
	}

[etpinard]

cc @plotly/plotly_js

[antoinerg]

Thanks for the fix @etpinard! Test is clear and simple 👌

Codepen with the fix: https://codepen.io/anon/pen/BMaNzV

💃