Plot positive AND negative change on the same axis

I'm designing a view for Android where the user can see a set of lists ordered in a tab bar. Each tab is represented by a block in a bar chart (+a label) which the user navigates through.

So far so good. My dilemma however is that each list can either have an aggregated positive or a negative change in value over time. The problem arrives with how to differentiate between positive change and negative change in the same bar chart without the chart compromising the space (eg. letting the bars go up / down depending on change).

I thought I could have the bars side by side stretching upwards depending on degree of positive or negative change, and then separate them by adding Blue for positive or Red for negative to the bar.

This is what I came up with:

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Problem is that it's not clear that the bars are communicated correctly to the user. I want there to be no doubt that the red bars depict a negative change and the blue bars depict positive change. Any bright ideas?

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