By default the tooltip uses the number formatter from the relevant axis to convert the value to a string. In this case as the axis is formatting its tick values to 0 decimal places, the values in the tooltip do likewise.
The simplest way to format the y-values to 2 decimal places would be to set the
labelFormatString property on the y-axis, so it displays values in the format you want. However, this would also affect the tick labels on the axis.
The other option would be to subclass
SChartSeriesCrosshairTooltip to override
setDataPoint:fromSeries:fromChart: to set the label text yourself: that way you can control how the numbers are formatted.