scripts: plot[mpl].py: Added --x/ylim-stddev for data-dependent limits

This adds --xlim-stddev and --ylim-stddev as alternatives to -X/--xlim
and -Y/--ylim that define the plot limits in terms of standard
deviations from the mean, instead of in absolute values.

So want to only plot data within +-1 standard deviation? Use:

  $ ./scripts/plot.py --ylim-stddev=-1,+1

Want to ignore outliers >3 standard deviations? Use:

  $ ./scripts/plot.py --ylim-stddev=3

This is very useful for plotting the amortized/per-byte benchmarks,
which have a tendency to run off towards infinity near zero.

Before, we could truncate data explicitly with -Y/--ylim, but this was
getting very tedious and doesn't work well when you don't know what the
data is going to look like beforehand.

scripts/plotmpl.py

@@ -132,6 +132,18 @@ def si2(x):
         s = s.rstrip('.')
     return '%s%s%s' % ('-' if x < 0 else '', s, SI2_PREFIXES[p])
 
+# find x/y limit based on a number of standard deviations
+def stddevlim(lim, xs):
+    # make a list, we need two passes
+    xs = [float(x) for x in xs]
+    if len(xs) == 0:
+        return 0
+    # calculate mean and stddev
+    mean = sum(xs) / len(xs)
+    stddev = mt.sqrt(sum((x - mean)**2 for x in xs) / len(xs))
+    # compute the limit as relative stddevs from the mean
+    return mean + float(lim)*stddev
+
 # we want to use MaxNLocator, but since MaxNLocator forces multiples of 10
 # to be an option, we can't really...
 class AutoMultipleLocator(mpl.ticker.MultipleLocator):
@@ -802,6 +814,8 @@ def main(csv_paths, output, *,
         height=HEIGHT,
         xlim=(None,None),
         ylim=(None,None),
+        xlim_stddev=(None,None),
+        ylim_stddev=(None,None),
         xlog=False,
         ylog=False,
         x2=False,
@@ -1029,6 +1043,8 @@ def main(csv_paths, output, *,
         define_ = define + s.args.get('define', [])
         xlim_ = s.args.get('xlim', xlim)
         ylim_ = s.args.get('ylim', ylim)
+        xlim_stddev_ = s.args.get('xlim_stddev', xlim_stddev)
+        ylim_stddev_ = s.args.get('ylim_stddev', ylim_stddev)
         xlog_ = s.args.get('xlog', False) or xlog
         ylog_ = s.args.get('ylog', False) or ylog
         x2_ = s.args.get('x2', False) or x2
@@ -1047,9 +1063,13 @@ def main(csv_paths, output, *,
 
         # allow shortened ranges
         if len(xlim_) == 1:
-            xlim_ = (0, xlim_[0])
+            xlim_ = (None, xlim_[0])
         if len(ylim_) == 1:
-            ylim_ = (0, ylim_[0])
+            ylim_ = (None, ylim_[0])
+        if len(xlim_stddev_) == 1:
+            xlim_stddev_ = (None, xlim_stddev_[0])
+        if len(ylim_stddev_) == 1:
+            ylim_stddev_ = (None, ylim_stddev_[0])
 
         # data can be constrained by subplot-specific defines,
         # so re-extract for each plot
@@ -1092,28 +1112,32 @@ def main(csv_paths, output, *,
             ax.set_yscale('symlog')
             ax.yaxis.set_minor_locator(mpl.ticker.NullLocator())
         # axes limits
+        x__ = (lambda: it.chain([0], (x
+                for dataset in subdatasets.values()
+                for x, y in dataset
+                if y is not None)))
+        y__ = (lambda: it.chain([0], (y
+                for dataset in subdatasets.values()
+                for _, y in dataset
+                if y is not None)))
         ax.set_xlim(
                 xlim_[0] if xlim_[0] is not None
-                    else min(it.chain([0], (x
-                        for dataset in subdatasets.values()
-                        for x, y in dataset
-                        if y is not None))),
+                    else stddevlim(xlim_stddev_[0], x__())
+                    if xlim_stddev_[0] is not None
+                    else min(x__()),
                 xlim_[1] if xlim_[1] is not None
-                    else max(it.chain([0], (x
-                        for dataset in subdatasets.values()
-                        for x, y in dataset
-                        if y is not None))))
+                    else stddevlim(xlim_stddev_[1], x__())
+                    if xlim_stddev_[1] is not None
+                    else max(x__()))
         ax.set_ylim(
                 ylim_[0] if ylim_[0] is not None
-                    else min(it.chain([0], (y
-                        for dataset in subdatasets.values()
-                        for _, y in dataset
-                        if y is not None))),
+                    else stddevlim(ylim_stddev_[0], y__())
+                    if ylim_stddev_[0] is not None
+                    else min(y__()),
                 ylim_[1] if ylim_[1] is not None
-                    else max(it.chain([0], (y
-                        for dataset in subdatasets.values()
-                        for _, y in dataset
-                        if y is not None))))
+                    else stddevlim(ylim_stddev_[1], y__())
+                    if ylim_stddev_[1] is not None
+                    else max(y__()))
         # x-axes ticks
         if xticklabels_ and any(isinstance(l, tuple) for l in xticklabels_):
             ax.xaxis.set_major_locator(mpl.ticker.FixedLocator([
@@ -1474,6 +1498,20 @@ if __name__ == "__main__":
                 dat(x) if x.strip() else None
                     for x in x.split(',')),
             help="Range for the y-axis.")
+    parser.add_argument(
+            '--xlim-stddev',
+            type=lambda x: tuple(
+                dat(x) if x.strip() else None
+                    for x in x.split(',')),
+            help="Range for the x-axis specified as a number of standard "
+                "deviations from the mean.")
+    parser.add_argument(
+            '--ylim-stddev',
+            type=lambda x: tuple(
+                dat(x) if x.strip() else None
+                    for x in x.split(',')),
+            help="Range for the y-axis specified as a number of standard "
+                "deviations from the mean.")
     parser.add_argument(
             '--xlog',
             action='store_true',