Support for the fallback to yaxis custom tickFormatter()

[alekperos]

Original version doesnt support the fallback to the yaxis custom tickFormatter(), for example, if value ticks across the Y axis are defined to be percentages, lets say, 50% instead of 50 as a value using tickFormatter, the valueLabels should have default support for that.

...if (series.valueLabelFunc)  val = series.valueLabelFunc({ series: series, seriesIndex: ii, index: i });
val = ""+val;

// enable custom formatting on valueLabels
if (plot.getOptions().yaxis.tickFormatter) val = series.yaxis.tickFormatter(Number(val), series);

if (val!=last_val || i==series.data.length-1) {...