Fix offsetGridLine behavior with a single data point

[nagix]

When a chart has only one data point, offsetGridLine: true does nothing. As you can see in the examples below, a grid line goes in the middle of the bar or point (1, 3, 5 and 7). These are inconsistent behaviours with other cases (2, 4, 6 and 8).

Current version: https://jsfiddle.net/nagix/9b6h3L2x

This PR fixes the issue so that offsetGridLine words for a single data point (1, 3, 5 and 7).

Proposed version (+ #4658): https://jsfiddle.net/nagix/wtpqLn5a

[benmccann]

This looks like a good improvement to me! Would it be possible to write a test case for this?

[nagix]

core.scale tests have been added.

[benmccann]

I think this draws a line on top of the axis? The axis looks extra dark in your images, which further reinforces my reading of this code. Perhaps it should return null instead and then not draw the line in that case

[nagix]

Yes, it draws a line on top of the axis, but this is not a special case as you can see in this official example (the y axis looks extra dark depending on the canvas width). This can be fixed by checking if the line and the axis border are overlapping, but wouldn't it be better to open a different PR?

[benmccann]

It'd be an easy change:

var xLineValue = getPixelForGridLine(me, index, gridLines.offsetGridLines);
if (helpers.isNullOrUndef(xLineValue)) {
    return;
}

[nagix]

This is a bit more complicated. If it returns here, not only the grid line but also the tick label will disappear as they are drawn together. That's why lineColor is set to transparent if the offset grid line is out of the range.

So, in order to avoid drawing the grid line over axis, we need to check whether each grid line is on the axis (this requires to check gridLines.drawBorder and options.position) and to set lineColor to transparent if needed.

[benmccann]

Ah, right you are. Ok, in that case, I agree with you that it's complicated enough that it would perhaps be better handled in another PR instead. Thanks for the clarification

[benmccann]

Avoid drawing line over axis

[simonbrunel]

@nagix we are considering a new patch version (2.7.3) so I wondering if we can merge this PR without #4658? or should we wait for both and release in 2.8?

[nagix]

There is no problem with merging this PR without #4658 as it can be handled separately.

[simonbrunel]

@nagix is this PR still valid? if so, can you rebase then I will merge it?

[nagix]

I have rebased and resolved the conflicts.

[simonbrunel]

Thanks @nagix