create a true "daily view" where whole graph is one day. Also, make it easy to link to this day.
AdminJacob N (Founder, WeatherSpark) commented
If you click on the "link"-link in the up right section you'll get a link that includes the current zoom level & range for the graph. If you bookmark that URL it'll take you back to the same view (may differ slightly depending on browser window size).
You can zoom in pretty far, though if you want truly single-day graphs you'll have to use the forecast tab: http://weatherspark.com/#!forecast;a=USA/CA/San_Francisco (and at this time there's no bookmarking other days than today for the forecast tab).