I need to make a bar graph showing a minimum, a maximum, and a calculated mean. Sort of like a high/low/close stock chart.

We call this type of graph a "floating bar graph."

Prism 5:

With Prism 5, you can make a floating bar graph from raw data. When you set up a Column or Grouped data table, choose a floating bar graph, with a line at the mean or median.

Prism 4

With Prism 4, you have to use a a trick.

Create a box-and-whisker graph. Enter the mean, min and max each twice, so six entries all together.

The box of a box-and-whiskers plot now goes from min to max (because min and 25  percentile are identical) and the whiskers are invisible. The center number in this type of graph can be either mean, median, or if a stock chart, close.