Custom ticks, category graphs and Prism 6.

"Category graphs" are graphs where the data table is grouped (rows have labels, but no X values) but the data looks like an XY graph with points. 

Prism 4 and 5 let you choose a category graph. The data table was Grouped ("two grouping variables") but the graph was XY. This means you could add custom ticks (now called "additional ticks and grid lines") to the axis. Since there were no X values, Prism simply used row numbers in place of X values. 

Prism 6 cleaned up this ambiguity. Category graphs are Grouped graphs. Both data table and graph are grouped. All the controls you want for symbol and error bars are right in the Format Graph dialog for Grouped data. Category graphs are no longer hybrid grouped-XY. 

But... this leads to a problem. There is no ability to add additional ticks and grid lines to the graph. The Format Axis dialog does not allow this for Grouped graphs. 

Workaround: From your graph, click the Change Graph Type button, or drop the Change menu and choose Graph Type. Choose an XY graph, and you'll be able to define additional ticks and grid lines.