I've wanted to plot curves for a while now, and finally modified my Kenwood power supplies to use voltage control for remote programming. I then grabbed a couple HP 59501B Isolated Programmable DACs off of eBay, and viola! The Excel file below is an RCA 12AX7 gray plate out of my stash that is most representative of a true 12AX7 out of the first dozen tubes I've tested. Hopefully I can start a new datasheet archive with many tube varieties.
While this is an Excel 2003 document, I was able to import into a Google doc. I presume Linux users (Geek et al) can use Open Office with success - let me know. This is more than pretty curves, as there's a schload of data you can fiddle with. I mean, you can print out a nice 8.5" by 11" chart to draw loadlines on, but you can also do all kinds of linear equations with the data. Starting at cell A27, you can see I've calculated Rp, Gm and Mu at one particular spot (the standard Va=250V, Vg1=-2V, Ia=1.2mA, Rp=62.5K, Gm=1600) for both halves. You can edit those formulas for any particular biasing point. For Rp, I would use the biasing B+ and 10V above and below. For Gm , I would use the bias grid voltage and the next step higher. In this case I used -2V and -1.5V.
http://www.diytube.com/curves/RCA12AX7.xls
Shannon