[plotter] make it possible to fit even if yerr_kind is None

plotter.py

@@ -1130,7 +1130,7 @@ class Plotter(Aggregator, BaseProcessor):
         Add an overlay : fit a curve, linear or powerlaw
         """
         if kind == "linear":
-            if yerr is None:
+            if yerr is None or np.sum(np.abs(yerr)) == 0:
                 (a, b, rho, _map_rule, stderr) = linregress(x, y)
                 self._log(
                     "Linear fit y = {} x + {} with R^2 = {} and error is {}".format(
@@ -1153,7 +1153,7 @@ class Plotter(Aggregator, BaseProcessor):
                     label = r"Linear fit with slope ${:.3g}$".format(a)
             plt.plot(x, a * x + b, label=label, **kwargs)
         elif kind == "power_law":
-            if yerr is None:
+            if yerr is None or np.sum(np.abs(yerr)) == 0:
                 (a, b, rho, _map_rule, stderr) = linregress(np.log10(x), np.log10(y))
                 self._log(
                     "Power law fit y =  x^({}) *  {} with R^2 = {} and error is {}".format(