Why can some error bars not be seen?

The Problem

You've created a graph and instructed Prism to plot error bars. But when you look at the graph, you notice that some of the error bars aren't visible. You may also notice the message shown here:

What's going on here?

Actually the reason that the error bars can't be seen is that they're too small and are hidden by the symbols that are plotted at the mean (or median) for the same data. When the error bars are very small - or when the summary symbols are very large - the symbol ends up hiding the error bars making it appear as if they don't exist.

Why does this occur?

Prism can graph error bars either from replicates you enter, or error values (SD, SEM, etc.) you enter. But sometimes, no error bar appears for certain points on XY graphs. The reason is simple: if the error bar is shorter than the size of the symbol, then the error bars won't be visible "behind" the symbol.

What can I do?

There are a few potential solutions that you can try (all of them are valid, and really just a matter of preference on how you'd like your graph to look).

1. Don't do anything! The error bars are small, and making other changes to your graph may simply not be worth the added benefit of displaying these small error limits.
2. Decrease the size of your summary (mean, median, etc.) symbol. If the size of the symbol that's covering the error bars is made smaller, then it's possible that the ends of the error bars will become visible.
3. Change the transparency of your summary symbol. By making the symbol fill (and/or border) color semi-transparent or fully transparent, the error bars will be visible behind the symbols (Note: this option won't work for Prism versions older than 10.5.0; see below).
4. [For Column or Grouped Graphs] Choose to plot the line and error "On Top" of the other graph objects. This option can be found in the "Lines" section of the Format Graph dialog on the Appearance tab (Note: this option is not available for XY graphs).

How older versions of Prism handle this situation

If you're using a version of Prism older than 10.5, then there's a slight difference in the way that Prism handles this situation. In these older versions, rather than drawing the error bars behind the symbols, Prism simply won't draw them at all if they're smaller than the summary symbol. This is true even if the symbol is clear. In other words, when the error bars are very small, Prism simply chooses not to draw them. The only method to have Prism show these error bars in this case is to:

1. Make the symbols smaller
2. Remove the symbols entirely
3. Choose to plot individual symbols for each replicate rather than plotting the error bars