fix: add boundary check just as a precaution

functime/_plotting.py

@@ -391,6 +391,9 @@ def get_num_rows(
     >>> get_num_rows(num_series=5, num_cols=2)
     3
     """
+    if any([num_series < 1, num_cols < 1]):
+        raise ValueError("Number of series and columns must be a positive integer")
+
     num_rows, remainder = divmod(num_series, num_cols)
     if remainder != 0:
         return num_rows + 1