Fix circles inherit alpha #2620

[patrick-elmquist]

Issue (#2620) :
When using multiple LineDataSets like the follows:

int solidColor = 0xFFFF00FF;
dataSet.setColor(solidColor);
dataSet.setCircleColor(solidColor);

int semiTransparentColor = 0x8A000000;
fadedSet.setColor(semiTransparentColor);

LineData data = new LineData(dataSet, fadedSet);

the circles in dataSet will rendered with the alpha from fadedSet (0x8A).

The reason for this is that mRenderPaint is not reset properly before drawing the circles. The first time drawCircles(...) is called the imageCache.fill(...) method is used where the color is set by mRenderPaint.setColor(set.getCircleColor(i)), restoring the alpha to 0xFF. The second time homever, imageCache.fill(...) is not called which means that mRenderPaint will use it's old color/alpha, which in this case is from fadedSet.

Test info:
To trigger the issue, add the following to LineChartActivity1:

final ArrayList<Entry> fadedEntries = new ArrayList<>();
for (int i = 0; i < count; i++) {

    float val = (float) (Math.random() * range) + 3;
    fadedEntries.add(new Entry(i, val));
}
final LineDataSet fadedDateSet = new LineDataSet(fadedEntries, "Faded");
fadedDateSet.setColor(0x42000000);

dataSets.add(fadedDateSet); // add the datasets

and launch the first item in the example app. Notice that the circles of DataSet1 is now semi-transparent, instead of solid black.

Solution:
This commit replaces mRenderPaint with null when drawing the circle bitmap. If a circleColor has been defined with a semi-transparent color, it will be drawn that way in the cached bitmap, hence the the bitmap itself does not need to be drawn with lower alpha.

[danielgindi]

Thanks!