We are currently working on that EXACT feature. It is a little complex, so we could not get it into 1.3, but it should be available in a future release.
BTW, the new .CSV Trend View export format that is part of 1.3 imports into Excel, which has WAY more graphing capabilities than Designer will ever have. See the attached screen shot for an example
The new format basically dumps ALL the values in a Trend View in a single line, at a specified heart beat (default is one data set per second). You can configure the heart beat by right clicking on a specific Trend View and select "Options for All Trend Views" (it's just the application level options for Trend View, you can actually configure just the CURRENT view from this dialog, which is what you will be doing).
Find the Export/Logging method group and select the first radio button
Log values for all elmeents on a specified time interval, and tweak the interval. The fastest interval is 100ms.
Make sure you have a fast enough PC, a lot of free disk space, then go back to the Trend View and hit the Record button on the toolbar (it is the red VCR Record looking button) and it will start logging IMMEDIATELY in that CSV form to a temporary file. Once you hit the STOP button (the red circle changed to a square VCR stop button when you hit the Record button), then it will prompt you for YOUR file name and save the data there (make sure you enter the extension .CSV so it will import into Excel nicely).
Usually all you have to do is double click on the .CSV and it will load it into Excel. Open Office's Calculate can handle .CSV also, but I don't know the graphing capabilities of Open Office.
You can also just EXPORT the current data set within a Trend View's history by hitting the Export button on the Trend View toolbar. It's the button with a red arrow pointing up on top of a small trend graph with the letters "Exp" at the bottom. So, use RECORD to START recording data NOW, and use EXPORT to export the current set of historical data for that Trend View.
You can do this all with the current 1.3. Attached to this post is a screen shot of Excel with some process data from the PID1.dmd example project where the temperature goes into High alarm (Y2 turns ON at 26:03.9 when the temperature SimPID.PV exceeds 300 degrees F). The log contains just 21 seconds worth of data from the PID view, logging one data set every 1 second (PID view is just a specialized trend view, but it supports the same functionality above). I'm not an Excel graphing guy, but you can do a lot of cool stuff with this. The log/export mechanism prepends the current Date/Time stamp to each line. Note that the first line in the file is the header which shows the data point names for that column.
.CSV text files can also be imported into most database packages.