BUG: make tsv 7 number summary tabluate base positions starting from 1 (

#167)

Previously the graph started counting sequence positions at 1 while the tsv started counting the same positions at 0. 

Fixes #110.

q2_demux/_summarize/_visualizer.py

@@ -63,8 +63,9 @@ def _subsample(fastq_map):
 def _compute_stats_of_df(df):
     df_stats = df.describe(
         percentiles=[0.02, 0.09, 0.25, 0.5, 0.75, 0.91, 0.98])
-    drop_cols = df_stats.index.isin(['std', 'mean', 'min', 'max'])
-    df_stats = df_stats[~drop_cols]
+    drop_rows = df_stats.index.isin(['std', 'mean', 'min', 'max'])
+    df_stats = df_stats[~drop_rows]
+
     return df_stats
 
 
@@ -189,10 +190,16 @@ def summarize(output_dir: str, data: _PlotQualView, n: int = 10000) -> None:
         scores = pd.DataFrame(quality_scores)
         if not scores.empty:
             stats = _compute_stats_of_df(scores)
-            stats.to_csv(
+
+            # ensure base positions begin from 1
+            stats_output = stats.copy()
+            stats_output.columns = range(1, len(stats_output.columns) + 1)
+
+            stats_output.to_csv(
                 os.path.join(output_dir,
                              '%s-seven-number-summaries.tsv' % (direction,)),
                 header=True, index=True, sep='\t')
+
             length_table = _build_seq_len_table(scores)
             qual_stats[direction] = stats