Refactor variable name x_i to avoid confusion with chromaticity

src/cassandra/tools.py

@@ -54,7 +54,7 @@ def _get_mean_values(
                 _logger.info("Values for PV: {}".format(pv))
                 try:
                     mean = sum(obj[1]) / len(obj[1])
-                    std = sqrt(sum([(xi - mean)**2 for xi in obj[1]]) / (len(obj[1]) - 1))
+                    std = sqrt(sum([(x_i - mean)**2 for x_i in obj[1]]) / (len(obj[1]) - 1))
                     print(output_str[output_format].format(name=pv, val=mean, std=std))
                 except ValueError:
                     raise ValueError("Probably too short time to get enough points")