Numbers charts using dates on axis

I have 4 sets of sample data I am looking to graph into one chart. The data is sample values by date the sample was taken. The data is on 4 tables, and the dates are not uniform. I would like to graph them all on one chart with a representative spacing of the data. My issue is that even though I have the x values in the table labelled as dates, they are spaced evenly, no proportionally on the chart. So I have the same spacing between 9/12 & 9/13 as I do between 9/13 & 9/19. It's throwing off my projections. How can I create a spacing on the x axis representative of the actual date taken and time between?