w/ Line graph with time on X and missing data

Hi,
I'm trying to do a line graph with months,year along the x axis and sales along the y axis. My data points on housing projects in a city.

My problem is - some months I don't have data for certain projects, but do have data for other projects. So when I graph this in a line graph (with points), the lines do not connect where I don't have a data point.

What I want is to just have a line connecting the two points that I do have and have a continuous line the entire length of time.

I can't just pick a mid points (or dumby point) b/c with some projects I have many points missing in a row.

See my attached file if I'm not being clear.
Please help, my job depends on this!!