How to select data for a chart, from another workbook?

I've created a chart that uses dates and figures from another worksheet in the same excel file.
The problem is that while this works fine, I run into problems in that I have dates for one year in each worksheet and then their will be twelve charts, each in their own worksheet. This makes the workbook too cluttered and difficult to navigate. So I wanted to have the worksheets with the dates in them in a separate workbook, and the charts in another workbook.
How do I reference the values in another workbook when selecting the data for the charts?

Okay I had to open them both and was able to use the Range selector. But now it always tells me there's an error whenever I change something on the chart.

Awesome working now thanks, even though it was set to prompt user, there was no prompt, so I put it on always do the links and it worked. Thanks again

Andy..

Thanks for tip. Really helped in explaining ti my friends!