Just discovered that there already is a delegate method for filling.

```/**
 *  Returns whether or not the area under the line should be filled
 *  
 *  Default: NO
 *
 *  @param lineChartView    The line chart object requesting this information.
 *  @param lineIndex        An index number identifying a line in the chart.
 *
 *  @return Whether or not the area under the line should be filled
 */
- (BOOL)lineChartView:(JBLineChartView *)lineChartView fillsAreaUnderLineWithIndex:(NSUInteger)lineIndex;
```