Long ago, Jeff Horsburgh's team at Utah State University created NWIS Analyst, which plots time series graphs for a given site, variable, and date range. http://water.usu.edu/nwisanalyst/
This page describes how a more generic version of that web-based application, the Time Series Analyst (TSA), could be incorporated into CUAHSI. In fact, it already has been incorporated, in a sense, because the graphing in ODM Tools is based on TSA. But TSA has utility in being a browser application, and could potentially be tied into DASH.
Jeff has put together functional specs for TSA. The specs indicate that TSA will be tied directly to databases. There was also talk of letting TSA work from web services instead of database connections. If this happened, TSA could replace the plotting functionality in DASH, because then TSA could work with NWIS and EPA (and other national data), in addition to local test bed databases.
TSA seems like a web version of ODM Tools, without the editing tab. So, what if we could split the interface for querying data (the Query tab of ODM Tools) and the interface for drawing the plots (the Plot tab)? The version of TSA that works with database connections would include both tabs, while the web service version would just work with the Plot tab. I'm assuming there's a clever way to make "tabs" or their equivalent for the web app.
On the plot tab, instead of a drop down list of sites and variables, maybe you just have text boxes, which are populated from the URL arguments defining what is being plotted.
Or, perhaps the Query tab is shown for both the database and the web service versions. There is a button called "Get Catalog" which gets sites and variables when clicked. So, if the user wants drop down boxes that are already populated with sites and variables, they push the button under the caveat that if they push it for NWIS, they'll be waiting a while. The drop down boxes will be the type that has a list of items to choose from, but also lets you type in your own item. That way, if the user didn't get the catalog, they could still type in the site code that they want even though the drop down list is empty.