Fix stacking together different series types (#1037)

[talc23]

Fix stacking together of different Series types.
for example, using this graph from showcase:
https://github.com/uber/react-vis/blob/master/showcase/plot/stacked-vertical-bar-chart.js

Added the last two lines to the showcase example:

        <XYPlot width={300} height={300} stackBy="y">
          <VerticalGridLines />
          <HorizontalGridLines />
          <XAxis />
          <YAxis />
          <BarSeries data={[{x: 2, y: 10}, {x: 4, y: 5}, {x: 5, y: 15}]} />
          <BarSeries data={[{x: 2, y: 12}, {x: 4, y: 2}, {x: 5, y: 11}]} />
          <LabelSeries data={[{x: 2, y: 10, label:"10"}, {x: 4, y: 5, label:"5"}, {x: 5, y: 15, label:"15"}]} />
          <LabelSeries data={[{x: 2, y: 12, label:"12"}, {x: 4, y: 2, label:"2"}, {x: 5, y: 11, label:"11"}]} />

Previously:

After fixing:

[talc23]

Running the tests on windows fail, what is the recommended setup for development?

[mcnuttandrew]

Hey @talc23,
This is looking good so far! I like that you have tests!! Awesome!
In your comment you say that you modified one of the show case examples, would you mind including that in this PR? (Maybe also write a test against that)

[talc23]

Sure, I'll add more tests.

Please notice that if you're using a clustered stack bars graph, you can't simply add LabelSeries to the graph since it doesn't have the 'cluster' attribute. Adding LabelSeries to such a graph will result the labels to stack over with no distinction of clustering. see this comment on the original issue - #1037 (comment)

[talc23]

Hi @mcnuttandrew,
Added the showcase example and a test.
Please review.

[mcnuttandrew]

nit: value -> row, i read this a couple of times as a really ineffective way to defined bars and labels

[talc23]

Not very familiar with Js, what should be a more effective way to do so?

[mcnuttandrew]

couple of code quality nits, but generally I approve. thanks for adding tests, code quality is important!

[mcnuttandrew]

Thanks for doing this!