How can I put a zero concentration point on my semi-log plot (e.g., competitive binding curve or dose-response curve)?

The logarithm of zero is not defined -- its mathematically impossible to plot zero on a log scale. Instead of entering zero, you can enter a low value (say -10 on the log scale), and then use custom ticks to label the graph correctly (so it is labeled "0" rather than "-10". Here is how to do it:

Double-click on the axis to open the Format Axes dialog.

At the bottom of the dialog, use the Additional ticks section to place a tick and label - 0 at the X value -10. Check to show only the additional ticks, not the regular ticks.

Now your graph has a log scale that appears to start at 0.

But isn't it cheating to enter a value other than zero? A discussion and alternative approach.

We also show the technique in these learning examples for Prism 3 and Prism 4.