Time of Day on Y Axis

I'm trying to create a chart that has the time of day on the y axis. I have a results table that looks like this:

Database      Time_of_Day
db_1          10:00
db_2          10:05
db_3          13:30

If I do a "chart max(Time_of_Day) by Database" I see results in the results table but nothing in any of the chart types. What am I missing here? How do I tell splunk to use a time scale for the Y axis?