traces with aliased events confuse the plotter

[JaviMerino]

@mdigiorgio reports that a trappy.FTrace object created with an events= parameter that aliases one that already exists confuse the plotter and doesn't let you plot what you want. For example:

trace = trappy.FTrace(trace_file, events=["thermal_temperature"])
trappy.LinePlot(trace, signals=["thermal_temperature:temp"]).view()
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-6-07a8dbdfd061> in <module>()
----> 1 trappy.LinePlot(trace, signals=["thermal_temperature:temp"]).view()

/home/javi/src/trappy/trappy/plotter/StaticPlot.pyc in view(self, test)
    172         if self._attr["style"]:
    173             with plt.rc_context(AttrConf.MPL_STYLE):
--> 174                 self._resolve(permute, self._attr["concat"])
    175         else:
    176             self._resolve(permute, self._attr["concat"])

/home/javi/src/trappy/trappy/plotter/StaticPlot.pyc in _resolve(self, permute, concat)
    236             width=self._attr["width"],
    237             length=self._attr["length"],
--> 238             title=self._attr['title'])
    239 
    240         self._fig = self._layout.get_fig()

/home/javi/src/trappy/trappy/plotter/PlotLayout.pyc in __init__(self, cols, num_plots, **kwargs)
     47         self._single_plot = False
     48         if self.num_plots == 0:
---> 49             raise RuntimeError("No plots for the given constraints")
     50 
     51         if self.num_plots < self.cols:

RuntimeError: No plots for the given constraints

However, trappy.thermal_trace.data_frame["temp"].plot() works. The aliased thermal_temperature confuses the plotter.