MAINT: PR 959 revisions

* fix up a `pandas` `FutureWarning`:

```
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]`
```

darshan-util/pydarshan/darshan/experimental/plots/data_access_by_filesystem.py

@@ -677,10 +677,10 @@ def plot_with_report(report: darshan.DarshanReport,
         height = num_cats
 
     plot_data(fig,
-              file_rd_series[sort_inds],
-              file_wr_series[sort_inds],
-              bytes_rd_series[sort_inds],
-              bytes_wr_series[sort_inds],
+              file_rd_series.iloc[sort_inds],
+              file_wr_series.iloc[sort_inds],
+              bytes_rd_series.iloc[sort_inds],
+              bytes_wr_series.iloc[sort_inds],
               filesystem_roots,
               num_cats=num_cats)