Use agg when testing plot methods (#170)

Author: jamesramsden-bh

Python_Engine/Python/tests/__init__.py

@@ -1,5 +1,7 @@
 from pathlib import Path
 import pandas as pd
+import matplotlib as mpl
+
 
 # identifier for all downstream processes
 BASE_IDENTIFIER = "PythonBHoM_pytest"
@@ -9,4 +11,7 @@ def get_timeseries():
     df = df.set_index(pd.to_datetime(df.index))
     return pd.Series(df["Value"], index=df.index)
 
-TIMESERIES_COLLECTION = get_timeseries()
+TIMESERIES_COLLECTION = get_timeseries()
+
+#use 'agg' for testing plot methods, as tkinter occasionally throws strange errors (missing component/library when component isn't missing) when the default backend is used only when using pytest
+mpl.use("agg")