This is a simple style change for the generated charts. It makes all the
charts the same size suitable for widescreen displays.

There are also some minor adjustments for line styles for better
readability especially when charts contain lots of data.

When set_xlim(), set_ylim() were called repeatedly on the same axes
after every plot, subsequent calls were ignored and only the first call
determined the axis limits.

This could result in charts where some data wouldn't be visible at all,
since it'd plotted beyond the smaller axis limit. To fix this, ensure
the axes limits are set only once -- when all the data is already
plotted.

Formerly extra files were drawn using the same colors as files withing
the limit and it lead risk of misinterpreting the data.

Colors are combined with markers to create more combinations.
Obviously nobody can really read 40 combinations in detail because the
points will overlap, but it is sufficient to detect outliers and that's
what I'm after.

Colors are combined with hatch to create more combinations.
Obviously nobody can really read 40 combinations in detail because the
points will overlap, but it is sufficient to detect outliers and that's
what I'm after.

Formerly extra files were silently ignored.

Each --group has user-supplied name, and line denoting average value,
and colored area denoting min/max range.

--rcode now also accepts mnemonic names + extra value "other"

pylint started to complain about length of the main() function.

New option --rcodes-above-pct N allows to draw only rcodes which are more
frequent than N percent of all answers. Thus --rcodes-above-pct 0 draws
all rcodes which have non-zero number of samples.

Small values like --rcodes-above-pct 0.1 % can also filter noise like
"five REFUSED answers in the whole dataset" etc.

Commit ad584cef changed color selection
so each line from a single JSON uses the same color. That works fine
when plotting multiple data sets but makes it harder to deep-analyze a
single dataset, e.g. when plotting all rcodes from a single run.

This commit changes color selection once again:
- When plotting multiple JSON files, all lines from one file use the
  same color.
- When plotting a single JSON file, each rcode line uses different
  color. Each supported rcode has a stable color.

Each rcode line is still denoted with a symbol introduced in
ad584cef.

Rcode lines use the same color as "summary" line.
Different rcodes are distinguished by markers derived from first letter
(s for SERVFAIL, n for NXDOMAIN ...) or rcode numbers for less common values.
NOERROR is displayed as 0 to distinguish it from n for NXDOMAIN.
Mathtext render of 0 also accidentally looks like O for Okay :-)

Query count intervals with 0 clients in them were not handled properly.
E.g. if some clients were in interval [0-10) and [100-1000) while interval
[10-100) had no clients in it the old script has thrown an exception.

Previously the loop ignored data points in range
10**int(log10(max(data)) to max(data).

Sometimes this range might have been empty already so not _all_
previous results are neccesairly invalid.

This is order of magnitude faster than dpkt implementation.

Add missing conversion from millisecods to seconds in QPS calculation.

Fixes #4