Adding percentage as a second y-axis on chart?

Hi all,

I bet this is embarassingly simple, but having done quite a bit of Googling and experimenting on my own, I haven't yet been able to figure it out.

I'm attaching an Excel file.

Basically I have 7 time based intervals with a count for each interval. I have also calculated the percentage for each count based on the total.

I've set this up as a bar chart showing the count at each interval.

What I'd like is to have the percentage included as well. I envisioned a secondary axis with percentage, but if there's some other way that's also a possibility.

Thanks in advance for any help.