Add 2nd data set to chart with new X values

Work have just upgraded me to Office 2010. I want to add data to a chart that has different x-values to the set it was created with.

Used to be I'd make a chart then go to add data, highlight the new columns, in the pop-up window tell excel that the first column is new x-values and the top row are data headings and bosh! job done.

In 2010 I add data and it makes me select where the heading is, what the new x-column is and where the new y-values are. OK. It's fiddly but I'll play along. Problem is it keeps picking up the data headers eg Time ns in the new X-column and won't use the new values.

Anyone else come up against this and found a way around it?

Please don't tell me to drag select the cells I want instead of column selecting because these data sets are thousands of values long.

I can't use any actual files as the data is confidential but I'll try and generate some spoof data sets sometime today.