It's been a year (and longer) since portfolio charting has been requested, so let me re-introduce that option.
Comparison charting is already available, and it's useful as far as comparing progress of individual stocks. However, that does not inform how the entire portfolio is performing.
The old Google Finance had this feature, and when they called it quits and I moved to Yahoo Finance, I fully expected to see the charting feature here. Major disappointment!
Yahoo Finance is a fine tool for investors, from the most casual to the most experienced day trader. Adding the charting feature would make it just about perfect.