```
total_group_sum.unstack().plot(kind='bar')
```

I am able to plot my bar graph, however now I want to draw a horizontal mean line over it. I try using `axhline()`

but I am getting an error:

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

How do I plot a horizontal mean line over my already existing bar graph?

```
total_group_sum.unstack().plot(kind='bar')
<IMAGE OF BAR GRAPH (I AM NOT ABLE TO POST PHOTO HERE)>
mean = total_group_sum.mean()
plt = total_group_sum.unstack().plot(kind='bar')
plt.axhline(mean)
ValueError: The truth value of a Series is ambiguous. Use a.empty,
a.bool(), a.item(), a.any() or a.all().
```

PS - I am using jupyter notebook

So, I tried recreating what I think is your problem, using random data:

I think your problem is, that this line:

returns the mean for every group:

Then, you call

Which wants to plot exactly one line, but you give it several. You can do two things:

either, plot the mean for every group:

or, calculate the mean of the means, and plot that: