On a graph showing the time course for these measurements, you'd like to show the variability in measurement time. Suppose you attempt experimental measurements at predetermined times, but have difficulty meeting those times precisely. And even then, the bars show variation or error in the Y-column variable. Prism 3 can only create horizontal error bars automatically for horizontal bar graphs. Click Change, and then Format Data Table. If you forgot to choose to include X error bars when you started your project, you can reformat your data table. Choose to make an XY graph with both vertical and horizontal error bars from the Prism Welcome screen. Prism 4 allows plotting X-variable error bars. Prism automatically graphs both horizontal and vertical error bars. Click theTable Format button in the upper left corner of the data table and in the Format Data Table dialog, click the box to add X-error values to the data table. You'll have to enter SD or SEM values directly.

Then enter your X-Axis error values in the appropriate table. Choose to make an XY graph, and check the box to create a data table with a column for you to use for your X error values. Prism 5 or later allows plotting X-variable error bars.

How to add error bars in graphpad prism