[Python] How do I plot hours and minutes when they are in different pandas columns?

I have a pandas' dataframe like:

hour minute cnt
0 0 904890
0 30 880374
1 0 848198
1 30 816488
2 0 791761


I would like to plot cnt using minute as minor ticks, and hour as major tick in matplotlib. I've tried several things but they didn't work. Here's one:

fig, ax = plt.subplots(figsize=(8,5))
df.plot(kind='line', y='cnt', ax=ax)
ax.set_xticks(df['minute'])
ax.set_xticklabels(df['minute'])


I'd like my x-axis to look like

|---+---+---+---+---+---+---+---|
0 15 30 45 0 30 45 0 15
0 1 2 ...


Thanks.

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