you need to create the line chart with 2 dimension in category and one measure. so that the party dimension comes as legend (different series for each party).
create a crosstab like below:
feb
| jan | mar | |
|---|---|---|---|
p1 | 12 | 10 | 12 |
p2 | 12 | 13 | 11 |
Right click on the block and select Turn To > Line chart.
You will get the chart like below:
