Comparison chart with empty cell

I have to create a graph that represents the electoral trend of many parties over time. in the X-axis I indicate the progress of time, in the Y-axis the percentage of votes. I would like to create a graph that contains as many lines as the parties I am considering. The problem I'm having is this: by default Excel considers the years when there is no data as 'zero'. This does not allow to show the trend over time and to compare the lines, because the graph always remains at zero, except for the years with the results. How can I avoid considering years without results as 'zero' but as 'non-data'?My table