I think your page items for start date and end date are not getting set like you think they are. Run the page from the development environment and, without changing anything, click the session link and see if they are set. I think your chart is running but, these are actually set to NULL.
To set them, create a Before Header computation for each of them to set them to SYSDATE and (SYSDATE + 30).
Thanks for the quick response. I'm an APEX newbie, though, so could you give me some hints on how to set the page items in the before header? The session page does indeeded show them as NULL before I hit go on page load and with values thereafter.
When you're in the editor, at the top of the page, you wee a section labeled Before Header. Expand this.
Inside, you see several options one of which is Computations. Right-click on this, and select Create. Choose your page item that is being set. Choose PL/SQL Function Body as the type. Here, enter:
That did it, thanks.
Edit: I had to put it under new instance instead of before header to be able to change the date, but that was what I needed.