i2p.android.base | I2P for Android | Android library

I'm still not sure I understand what you're asking but I'll attempt to give some general information about what is happening in the code you posted.

Essentially, _ratePlot.setRangeValueFormat(new Format()...) is setting a custom Format to be used when rendering the range values along the edge the XYPlot. Androidplot will call the format method on this anonymous inner class, passing in each range value as a Number in the obj parameter. You have to cast it back to a Number inside this method because Format class is meant to be generic. This is done in the first line of the method:

double val = ((Number) obj).doubleValue();

Once you have this value, it's up to you to decide how you want to format it. The rest of the method just does some formatting and returns the formatted result as a StringBuffer.