Perhaps a few more ideas why I like combined graphs so much:
Whenever we discuss weather, we usually can't look at isolated parameters (wind speed, direction, temp, hum etc.) as they change all at the same time, impacting our local conditions. I've always been fascinated by how those parameters interact and I love to watch and discuss their correlation in distinct weather events. I did that a lot with my old site and its weather blog. The all-in-one meteoplug graph was a favourite when it came to illustrating those weather events. Plus, it was the most popular chart with the site visitors.
Another reason why I like combined graphs is that the correlation of changes displayed allows me to make short-term forecasts. Knowing the weather around here and how it usually (!) changes, I know from experience what to expect. Combined graphs make it easier to spot significant changes.
I've set up a test version of meteoplug (again) and it turns out a rather nice chart, but it's a 500k monster. Plus, I wouldn't know how to use this in a block or custom page.
http://www.wachtendonkwetter.de/daychart.html
Perhaps a handy block or plugin will come along some time. Until then, I'll keep on tinkerin' with what's available and see where it takes me.
Cheers
Chris