Date mismatch when using external axes object

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I have the following dataframe that contains ohlc data:
`candles.head()`
I want to plot the data using an external axes object and then add additional stuff on the chart using matplotlib. The problem is that there seems to be a mismatch in the way mplfinance and matplotlib handle dates. For example, the following code:
```python
fig = mpf.figure(style='yahoo')
ax = fig.add_subplot(1,1,1)

mpf.plot(candles, ax=ax)

ax.plot(candles.index, candles.open)
```
should overlay a line connecting all the opens of the candles. However, the result is this:
Am I doing something wrong? Thank you

Start with the external-axes example using mpf.plot(candles, ax=ax) followed by ax.plot(candles.index, candles.open), and trace how each call handles the dataframe's dates. Reproduce the mismatch with the supplied example; done means the additional line overlays the corresponding candles without requiring callers to manually convert dates.

Written by the indexing model from the issue text.