#52 ClusteredBarChart should also plot values=0

[Thach Nguyen]

When ValueLabelRenderer is used to show data point
labels on clustered bar charts, any labels with values=0
end up placed on top of previous non-zero labels.

Ex: data={{14, 5, 1, 1}, {0, 1, 2, 3}}

The first bar with value 14 will have label 0 plotted on top
of the digit "4". This is because methods verticalPlot()
and horizontalPlot() take into account not to plot any
values that are 0. This is inaccurate for plotting because
zero values are still plot-able values.

The workarount is to take out the flag:
if( iAxisChartDataSet.getValue( j, i ) != 0.0d ) and plot
everything.

[Thach Nguyen]

ClusteredBarChart.java with plotting value=0 fix.