Issue 1764: Add default colors for Pie Charts

[ds17f]

I wrote a simple fix for default colors on the pie chart.

Here's the basic idea:

1. Create 2 new arrays in the global.defaults
   • segmentColorDefaults
   • segmentHighlightColorDefaults
2. Populate each of these arrays with a list of colors (ie: #AABBCC)
3. Update the "addData" routine on the Pie Chart so that, if a color is not specified, a color and highlight color is chosen from the defaults
   • For simplicity, the index of the item which is being added is used to look up the corresponding color in the list of default colors
     • I suppose I could use a stack of some sort and pop off the top each time a color is used, but that's more than I'm up for at this moment

Users can then override the list of default colors/highlights at a global level should they feel the predefined ones are not adequate/enough. Failure to define colors will simply roll over to the defaults.

[etimberg]

Might be safer to use index % Chart.defaults.global.segmentColorDefault.length
It is unlikely that there would be more than 15 segments, but someone could change the array to a shorter one.

[ds17f]

Definitely a good call. I added a commit to revise this.

[etimberg]

Overall looks pretty good. Did you see https://github.com/nnnick/Chart.js/blob/master/src/Chart.Doughnut.js#L76 as well?

[ds17f]

@etimberg I hadn't seen that. I'm not sure I get exactly what that's doing without context and I'm not sure if it needs to be patched to support this change.

Forgive me, JS isn't my primary language. I'm happy to finish out the change but I need a little guidance if you would.

Thanks

[ds17f]

I'm going to take a look at the default color set too. I kind of just picked a random set but I'm sure I could choose a better set of colors/highlights that make them go better together.

[etimberg]

I don't think it needs to be patched to support it. It simply adds a random colour if one is not added. Let me know when you've settled on the final colour set and I'll merge this. Thanks for the PR 😄

[ds17f]

@etimberg I chose to use 12 colors instead of the 15(?) I had before. I read somewhere that that's the most an eye can process. I also tried to pick good main/highlight colors. It looks good to me. You can merge when you feel it passes muster.

You're welcome for the PR. Thanks for the excellent project! It is the tool that I needed to make my work beautiful.

[etimberg]

Tested, looks nice! Great colour choices!