Add error margin for detecting if a point or line is in the chartArea

[nagix]

Many issues about points/gridline at the top edge of the chart area being cut have been reported. The issue is caused by a decimal round-off error when detecting if a point or line is in the chartArea. This PR adds an err margin, which is already implemented for point clipping at the right and bottom edges.

v2.7.3: https://jsfiddle.net/nagix/z1bx459c/ (To reproduce the problem, you may need to resize the container as this is caused by decimal rounding)

v2.7.3 + This PR: https://jsfiddle.net/nagix/yv6emrcf/

Fixes #4579
Fixes #4790
Fixes #5493

[nagix]

Maybe 1% margin is too high because it allows 10px off the chart when a border is at 1000px from top/left. 0.01% should be fine.

[simonbrunel]

Do you think that using a percentage is the best solution or should we express errMargin in pixels instead? (e.g. < left - errMargin)?

[benmccann]

@nagix you may also be interested in the pending PR to fix point clipping on the left and right: #5190

[nagix]

Since we don’t need high precision, a margin in pixels seems to be easy to understand.

[nagix]

I would like to address #5190 separately because it is about a layout issue while this is about a rounding error which can be easily fixed.

[nagix]

Replaced errMargin with epsilon, which is used in the other place.

[benmccann]

Agreed that we should address #5190 separately. It would be useful to get your review on it since you're currently working on this part of the code and know it better than I do. It'd be great to get that one merged too if you think it's a good solution

[nagix]

Do we need to fix the codeclimate issue?

[benmccann]

No, I don't think so

[simonbrunel]

Nice work @nagix. Does 0.0000001 enough in all cases, how did you chose that value?

[nagix]

It’s from your #5597 ;) No one can tell if this distance from the border is inside or outside, and rounding errors usually happen around 10^-15 or 10^-16

[qreeves]

Well done @nagix and thanks for the fix!

[simonbrunel]

@nagix I think this PR breaks some cases: if you test this example (code), you will notice that the last points disappear with master (maybe epsilon is too low?):

[nagix]

@simonbrunel This is related to #5797 and happens only when a category scale is used. As explained in #5797, getPixelForValue rounds the offset value and the x value of the point may exceed chartArea.right depending on whether the decimal part of scale.left is less than 0.5 or not.