I choose to graph confidence or prediction bands with nonlinear or linear regression, but they don't appear on the graph.

Prism will not plot confidence or prediction bands in several situations:

• If the best-fit value of a parameter hits a constraint, the fit is unlikely to be useful. Prism does not plot confidence or prediction bands, because they would almost certainly be misleading.
• If the results of nonlinear regression are ambiguous, the confidence or prediction bands would be super wide, maybe infinitely wide. They would not be useful, so Prism does not plot them.
• If you choose robust nonlinear regression, Prism does not compute confidence or prediction bands, as it cannot compute standard errors or confidence intervals of the parameters. 
• The fit is perfect. If the sum-of-squares is 0.0 and R² is 1.0, it is not possible to compute or interpret confidence or prediction bands.
• If the fit is interrupted, confidence and prediction bands are not computed.

Prism 4

If you chose nonlinear regression and chose to weight your data points unequally, Prism 4 cannot plot confidence or prediction bands. Prism 5 and later can.

Prism 4 can plot 95% confidence (or prediction) bands of the best fit curve. If you check this option when you first fit the curve, Prism will superimpose the confidence or prediction bands on your graph. If you use an early build of Prism (before 4.02 and 4.0b), you should update. If you cannot update, follow these steps:

1. Click Change analysis parameters, and check the option to view the confidence or prediciton bands.
2. Go to the graph. With Prism 4.02 and 4.0b, you will see the bands. With earlier releases, you won't see the bands yet so follow the rest of these steps.
3. Click Change..Symbols and lines
4. Drop the list of data sets and choose the curve fit. Note that Prism sees the curve fit as just another data set to plot.
5. Check "show error bars".
6. Choose the "- - - -" or "___" style.
7. Choose a color and thickness