This is such a common problem, it has always surprised me that most spreadsheet programs don't handle it well. I think Excel only started to handle it at version 2003 or so, and it still doesn't do it quite right. There
is a technique you can use that ought to work in any spreadsheet program. That's the good news. The bad news is that I got so frustrated trying to implement it in Numbers on my iBook G4 that I gave up before I could assemble a full example for you.
Here's the basic trick: Don't try to make a short table of dates and their corresponding values. If you had data to plot for only 6 dates out of the year, you would, normally, make a table six rows long & try to make a chart based on that. Don't do that. Instead, make a table that is 365 rows long -- one row for every date of the year. Then fill in your data on the appropriate rows, and make a chart form this new, long, sparsely-populated table.
In Numbers, I found that this works pretty well if you want a bar chart, or a scatter-type chart. Making a line chart is tougher, since (as has been noted) Numbers won't connect the dots on a scatter plot. The best answer I can think of is to generate another column for the table, where you would interpolate the values between the user-specified points. I tried to do this, but got fed up with frequent spinning beach balls of death. Apparently this task is too much for Numbers, or at least too much for my system.
Fond as I am of Numbers, this sort of limitation is one of the reasons I'll wait for version 2.0 before committing any serious work to the program.
