Added gradient line drawing to LineChartRenderer

[jjatie]

Reimplemented #649 for modern Swift and Charts

[natacodes]

Excited to see this re-implemented for a recent version of Charts. Thank you, @jjatie 👍🏻
Could you provide an example usage? I pulled your branch, added the following code but I don't get any gradient:

        set1.isDrawLineWithGradientEnabled = true
        set1.gradientPositions = [40.0, 120.0]

EDIT: I was missing set1.setColors([color1, color2, color3], alpha: 1.0)

I still can't get a nice gradient line. Let's say I want everything below 50 to be red, between 50 and 100 to be yellow and 100+ to be green. What values should I put it setColors and gradientPositions?

I tried this:

        // values: 50, 300, 50
        set1.mode = .linear
        set1.isDrawLineWithGradientEnabled = true
        set1.setColors([.red, .green, .yellow], alpha: 1.0)
        set1.gradientPositions = [80, 130, 200]

Result:

Also it might be a good idea to expose gradient colours as an open variable.

A few things I noticed:

• It's only implemented for cubicBezier and linear, missing cubicHorizontal and stepped
• isDrawLineWithGradientEnabled is set as { get set } instead of following the convention used in this library which is having drawLineWithGradientEnabled -> { get set } and isRawLineWithGradientEnabled -> { get }
• With certain values in data set I'm getting Thread 1: Fatal error: Can't form Range with upperBound < lowerBound

[natacodes]

_xBounds.max and _xBounds.min can be zeros when there's only one data entry and this for is crashing the app with Fatal error: Can't form Range with upperBound < lowerBound

[natacodes]

It needs if dataSet.entryCount > 1 somewhere.

[liuxuan30]

@natacodes thanks! good to have you help reviewing and providing feedback.
We plan this for 4.0 feature.

[jjatie]

@natacodes Thanks for the feedback. To be honest, I blindly reimplemented the solution from #649. I just wanted to get the ball rolling again on what I thought was a good feature. I would like to have an API that is unambiguous as to what values to provide as you mentioned.

I will hopefully have time in the next week or so to more closely evaluate and test the solution. In the meantime, I encourage you to PR any suggestions you have for the implementation into this branch.

[natacodes]

I encourage you to PR any suggestions you have for the implementation into this branch.

@jjatie I tried, but it's a little bit too complex for me at this time.

[jjatie]

@natacodes No problem. This will take a little time for us to get around to, but I hope you are able to continue provide feedback when we do. Thank you.

[liuxuan30]

I actually wanted to try this out in my new app after we take care of others.

[larryonoff]

@jjatie just want to check that this line is correct. It iterates over dataSet.colors, but always takes color at index 0.

[jjatie]

    for i in dataSet.colors.indices
    {
        cColor = dataSet.color(atIndex: i)

[larryonoff]

I think this PR can be closed now, since PR #3415 is based on this one and it was just merged into branch 4.0.0.

[Ferszt]

Hey guys, can this functionality be used in production now?

[larryonoff]

@Ferszt please use branch #4.0.0. The functionality already there.