Invisible Trendline?

First of all, hello - I am new to this forum, and this is my first post.  Hopefully someone might have some insight on this.

I have a scatter plot containing 12 series, originally created several years ago in Windows XP (Excel 2003?).  I got a new PC two weeks ago (Vista w/Excel 2007), and after copying over the file and opening it on the new machine, two linear trendlines that existed in the original file no longer appeared in the plot field.  They do show up as selectable elements, but that's all.  I deleted both trendlines, and then tried to add one back - again, nothing in the field of the plot (although it does show up in the legend).  I checked the line style, color, etc to make sure it was something visible, but no luck.  For what it's worth, I can get the formula to appear, but that's it.  I happen to have another worksheet in the same file, and the trendline in that shows up just fine.

This is my first time using 2007, so it may simply be my ignorance with the new version.  The file is currently being saved as an Excel 97-2003 Workbook (in the hopes that it will be compatible with my PC @ work).

 Thanks in advance for your suggestions.

Ken,

Can you post the offending workbook?

Another question, if you save it in 2007 in 97-2003 format, is it visible again in those vresions?

 With your mouse over the data points, did you right click your mouse, then select the "add trend line" option? I don't know if this feature is still around in Excel 2007, but I know it works in Excel 2003.

 Thanks for the suggestion - yes, I've tried that, but I get the same results as before (i.e., no trendline on the plot)

I'm attaching the file to an earlier response

 File is attached.  Worksheet 'runplot2' has numerous series, of which two (2007 and 2008) had trendlines shown in the body of the plot in the old version.  Now, nothing.

 I am intimidated by your running and biking exercise routine that you have stuck with for at least 10 years. Even though I don't have Excel 2007, I do have the conversion utility.

As I do not have the Excel 2007, I can't provide you with the solution you are seeking, however, you do have a lot of data that you are portraying in one space. What I would suggest that you do is to have a separate graph for each year with a y axis that is the same scale for each year. This way, you can have three graphs on 1 page to show each year. This would be easier to segregate the data for each year. What is your overall objective, and is your current system of how you portray your data accomplishing your goal?

 I've constructed it this way because I want to compare from year-to-year, and the easiest way for me to do that is to have one plot.  I wouldn't have expected that I was running into any limit with 2007, but then again, I've only been using it for ~2 weeks.  As a workaround, I'm creating a new plot, starting with my CY08 data, and adding new series in reverse chronological order.  So far, the trendline for 2008 is visible, and I will see how far I can go (the more recent data is more relevant than the older data, but I would still like to see it).  I've certainly made individual plots at work from files with 10,000+ rows of test data, so this would seem pretty wimpy by comparison.

Thanks for your suggestions.