Add a Utility, selector or slicer to filter individual lines on line graph or add a second 'ghost' line.
Based on historical data, it would be nice to see what would have been. For instance, if we are reviewing year to date figures and revenue by month on line graphs; we would have the ability to edit the interaction on individual lines. Customer A, B, C & D total our overall Revenue, but what would it have looked like if we didn't have customer B? This can be used for several other approaches to data analysis. Similar to forecasting; however, we are looking at historical data to quantify the importance of a variable (aka customer B). We have the ability to edit interactions on the whole visual, but this would add a second “ghost” line or give the ability to slice/filter/modify a second line already on the graph. It is also based on visualizing data such as 'excluding' data from bar charts. Lastly, adding this on the visualization side would decrease the size of our data source; in which, we would normally modify, group or call data in order to get the same comparative result.