Fix the issue that the topmost tick label and the bottom of the chart area are cut off with a radial scale

[nagix]

When the legend is hidden in a polarArea chart, the topmost tick label is cut off. The bottom of polarArea, pie and doughnut charts is also cut off depending of the canvas size.

The first problem happens because the height of the tick label is not considered for the padding calculation of the chart. This PR adds a few lines to the radialLinear scale for giving a hint of the top padding.

The second problem is caused by Math.round used to calculate the center point and radiuses. This should be Math.floor otherwise the chart area becomes larger than the canvas.

Chart.js 2.7.3: https://jsfiddle.net/nagix/zm8gvnrs/

This PR: https://jsfiddle.net/nagix/t46b127j/

Fixes #5761

[simonbrunel]

scale.update() is (unfortunately) very expensive, why do we need to call this method here while we call it already multiple time during the layout? Is the one at the step 9 not enough?

[nagix]

I'm trying to take the same approach as the other vertical axis - it sets the paddingTop property at the end of fit if the axis and ticks are visible, and the step 4 picks it via getPadding. For chartAreaBoxes, this is the first time for calling update.

Chart.js/src/core/core.scale.js

Lines 482 to 483 in f6d9a39

	me.paddingTop = tickFont.size / 2;
	me.paddingBottom = tickFont.size / 2;

In order to get paddingTop, we need to calculate drawingArea which is based on the sizes and positions of point labels in the chart.

[simonbrunel]

I understand, but I'm very worry about consequences of calling update() on scales another time because it's already an issue (#5389). @etimberg any idea?

[etimberg]

There are risks to calling update() again. In an ideal world, I think the update algorithm would iteratively call update() on everything until the boxes either don't change or change only by a small amount.

The current algorithm tries to approximate that by calling update() twice. The reason that chartAreaBoxes were only updated once is that I thought that the size should be dependent on everything else. i.e., the chartAreaBox is whatever is left over after the other parts have run through the fitting algorithm. That may have been a bad decision.

An alternative fix might be to change the calculation on https://github.com/chartjs/Chart.js/blob/master/src/scales/scale.radialLinear.js#L149 to include half the font height

[nagix]

@etimberg Thanks for your advice. I changed the calculation of largestPossibleRadius and yCenter so that update() is called only once.

[simonbrunel]

@nagix this PR needs a rebase following the merge of #5858

[nagix]

Rebase done

[simonbrunel]

I should have run unit tests on my machine before, but same issue as #5842 (Firefox only)

[nagix]

I will fix this and verify on my Windows PC, too.