TypeError: object of type 'int' has no len() - renko with return_calculated_values and mav

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Hello,

Firstly i'd like to say this is an excellent library, very very good indeed, and I really appreciate the work done here.

I am using the `return_calculated_values` feature so I can test (and hopefully add to the plot) my own studies that are based on the renko data.

I am using python 3.7 on Google Colab, the following code works perfectly, and `retvals` is populated:

```python
retvals = {}
mpf.plot(df, type='renko', return_calculated_values=retvals,
title=f'ADAUSDT / Renko Chart (1m base)',
renko_params=dict(brick_size='atr', atr_length=10),
style='yahoo', figratio=(35,12),figscale=1.8
)
```
However, adding `mav=10` or `mav=(10)` generates an error:

```python
retvals = {}
mpf.plot(df, type='renko', return_calculated_values=retvals, mav=(10),
title=f'ADAUSDT / Renko Chart (1m base)',
renko_params=dict(brick_size='atr', atr_length=10),
style='yahoo', figratio=(35,12),figscale=1.8
)

TypeError Traceback (most recent call last)
in ()
5 title=f'ADAUSDT / Renko Chart (1m base)',
6 renko_params=dict(brick_size='atr', atr_length=10),
----> 7 style='yahoo', figratio=(35,12),figscale=1.8
8 )

/usr/local/lib/python3.7/dist-packages/mplfinance/plotting.py in plot(data, **kwargs)
483 if config['mav'] is not None:
484 mav = config['mav']
--> 485 if len(mav) != len(mavprices):
486 warnings.warn('len(mav)='+str(len(mav))+' BUT len(mavprices)='+str(len(mavprices)))
487 else:

TypeError: object of type 'int' has no len()
```

Dependencies:
```
mplfinance in /usr/local/lib/python3.7/dist-packages (0.12.7a17)
pandas in /usr/local/lib/python3.7/dist-packages (from mplfinance) (1.1.5)
matplotlib in /usr/local/lib/python3.7/dist-packages (from mplfinance) (3.2.2)
numpy>=1.15.4 in /usr/local/lib/python3.7/dist-packages (from pandas->mplfinance) (1.19.5)
pytz>=2017.2 in /usr/local/lib/python3.7/dist-packages (from pandas->mplfinance) (2018.9)
python-dateutil>=2.7.3 in /usr/local/lib/python3.7/dist-packages (from pandas->mplfinance) (2.8.1)
pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in /usr/local/lib/python3.7/dist-packages (from matplotlib->mplfinance) (2.4.7)
kiwisolver>=1.0.1 in /usr/local/lib/python3.7/dist-packages (from matplotlib->mplfinance) (1.3.1)
cycler>=0.10 in /usr/local/lib/python3.7/dist-packages (from matplotlib->mplfinance) (0.10.0)
six>=1.5 in /usr/local/lib/python3.7/dist-packages (from python-dateutil>=2.7.3->pandas->mplfinance) (1.15.0)
```

Thanks

Start by reproducing the Renko chart call with return_calculated_values and mav=10, then inspect plotting.py around line 485 where mav is passed to len(). Compare the scalar and tuple cases and verify that the corrected behavior still handles the reported Renko configuration without raising TypeError.

Written by the indexing model from the issue text.