Combo charts that don't mess your unedited data

Hi guys I wonder if you can help?

I'm not sure if I am messing up here or something screwy is happening. I've asked a few colleagues for their opinions (and lots of web browsing) but so far I haven't gotten to the bottom of it.

I have a chart with multiple data sets that I want to display in a combo of smooth scatter and area under the smooth scatter. Here is my original chart:

Original Chart.png

I'd like to have the area under the orange line filled. When I change my chart type to combination and select 'Area' for that particular dataset, my chart now looks like this:

Fill Area.png

Note the blue lines, my other data sets have changed also the orange area is not as smooth as the scatter line, but the x/y axis remain the same (as does the data selection). I also want this red line as an area:

Fill Area-red.png

If I do the same procedure the red data set moves on the chart!

Fill Area-red-move.png

It's jumped ahead in time. Same dataset used, same axis.

Am I doing something stupid?