Bug Report:

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Bug when using box_size = 'atr' in pnf charts in mplfinance : FutureWarning: Series.__getitem__ treating keys as positions is deprecated

OS Win 11 Pro 24H2 Desktop
IDE Visual Studio Code 1.96.2
Python 3.12.8
mplfinance 0.12.10b0

When using the 'atr' box_size parameter in creating pnf charts in mplfinance the `_utils.py script` throws this warning

\mplfinance\_utils.py:129: FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]` high = highs[i]
\mplfinance\_utils.py:130: FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]` low = lows[i]
\_utils.py:131: FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]` close_prev = closes[i-1]

This is the responsible function:

```
def _calculate_atr(atr_length, highs, lows, closes):
"""Calculate the average true range
atr_length : time period to calculate over
all_highs : list of highs
all_lows : list of lows
all_closes : list of closes
"""
if atr_length < 1:
raise ValueError("Specified atr_length may not be less than 1")
elif atr_length >= len(closes):
raise ValueError("Specified atr_length is larger than the length of the dataset: " + str(len(closes)))
atr = 0
for i in range(len(highs)-atr_length, len(highs)):
high = highs[i]
low = lows[i]
close_prev = closes[i-1]
tr = max(abs(high-low), abs(high-close_prev), abs(low-close_prev))
atr += tr
return atr/atr_length

```
This warning is suppressed when you modify the code like so

```
high = highs.iloc[i]
low = lows.iloc[i]
close_prev = closes.iloc[i-1]
```

This however creates a new problem if you subsequently run a Renko chart, you get this error

LocalCache\local-packages\Python312\site-pa
high = highs.iloc[i]
^^^^^^^^^^
AttributeError: 'numpy.ndarray' object has no attribute 'iloc'

To fix this I created a small helper function

```
def safe_indexing(data, index):
if isinstance(data, pd.Series):
return data.iloc[index]
else:
return data[index]
```

and placed it in _utils.py
and updated _utils.py

```
high = highs[i]
low = lows[i]
close_prev = closes[i-1]
```

to

```
high = safe_indexing(highs, i)
low = safe_indexing(lows, i)
close_prev = safe_indexing(closes, i-1)
```

Now both charts render with no messages

Start in _utils.py at _calculate_atr and trace how its highs, lows, and closes inputs are supplied for PNF and Renko charts. Reproduce both chart cases from the report, then verify that the warning is gone for PNF and the Renko path no longer raises the reported ndarray attribute error.

Written by the indexing model from the issue text.