paranoia for plot coverages, bugfix

src/mbf/genomics/regions/plots.py

@@ -55,7 +55,7 @@ def store(
         coverage, gr_name=gr.name, sample_name=sample.name, extend_reads=extend_reads
     ):
         key = gr_name, sample_name, extend_reads
-        if key in coverage:
+        if key in _coverage:
             raise ValueError("Douplicate key usage in coverage cache")
         _coverages[key] = coverage