Prism 6 mac bug: Can't alter plot-a-function results created by Prism 5.

Steps to reproduce:

1. In Prism 5, create an analysis "Create a family of theoretical curves". That is the same as what Prism 6 calls "Plot a function". 
2. Save the file in Prism 5.
3. Open with Prism 6.0a Mac.
4. Open the parameters dialog, make a change and click OK. 

What happens: Nothing. Clicking OK does not close the dialog and recalculate the analysis. 

Reason for the problem: Prism 6 now allows you to choose to also plot the derivative of the function and its integral. To plot the integral, you need to enter the starting value (Y at the first X value). If you don't choose to plot an integral, this value has no meaning. Nevertheless, Prism 6 Mac won't let you leave the dialog without a value entered into that spot.

Workaround: 

1. Check the option "Plot the integral of this function".
2. Enter a value (0.0 works fine) for the beginning value.
3. Uncheck the option "Plot the integral of this function".
4. Click OK to the dialog, which will now close properly.

We'll fix in 6.0b. It is not a problem with Prism 6 Windows.