Performance Enhancements #29 - candle chart

[dorsoft]

performance enhancement for the candle chart.

[danielgindi]

How exactly does this increase performance?
This actually makes the renderer loop over more labels than needed.
It also made scrolling jerkier when having 2.5K points

[dorsoft]

1. I'd forgot to make the same change in drawDataSet(), I've pushed the fix to the PR. please pull and run the candle demo with 2500 candles.
2. dataSet.entryIndex becomes slower as the chart is scrolled from left to right for datasets > 300.
   I've added time measurements logs and ran the candle demo with 2500 candles (iPhone 5) and scrolled the chart all the way to the right. I've found out that the 4 lines of code with dataSet.entryForXIndex and dataSet.entryIndex take about 70-90ms to complete and are called about 5-7 times a second i.e. (5 X 2 X 70ms) to (7 X 2 X 90ms) = 700ms to 1260ms per second are wasted.
3. The render loops over the same number of labels and candles as:
   var entryFrom = dataSet.entryForXIndex(_minX)
   var entryTo = dataSet.entryForXIndex(_maxX)
   var minx = max(dataSet.entryIndex(entry: entryFrom!, isEqual: true), 0)
   var maxx = min(dataSet.entryIndex(entry: entryTo!, isEqual: true) + 1, entries.count)

is always equal to

var minx = max(_minX, 0)
var maxx = min(_maxX + 1, entries.count)

You can test this easily by adding some testing code like:

var minx2 = max(_minX, 0)
var maxx2 = min(_maxX + 1, entries.count)
if (maxx != maxx2 || minx != minx2) {
println("maxx2 or minx2 are different ! ! !")
}

[danielgindi]

This introduces only a very minor improvement, even for 2500. You have probably measured in an un-optimized compilation. Anyway, I've tested and did not find any side effects.