Getting an error when base an indicator on a calculation of two other indicators from a second data feed:

```
import backtrader as bt
import backtrader.indicators as btind
class myStrategy(bt.SignalStrategy):
def __init__(self):
self.ema_day = btind.EMA(self.data1, period=15)
self.kama_day = btind.KAMA(self.data1, period=25, fast=3, slow=30)
differ = self.ema_day - self.kama_day
self.roc = btind.ROC(differ, period=6)
def next(self):
pass
data = bt.feeds.GenericCSVData(
dataname='./data/VXX-15m-2017-10-01us.csv',
dtformat='%Y-%m-%d %H:%M:%S',
openinterest=-1,
timeframe=bt.TimeFrame.Minutes,
compression=15
)
data_day = bt.feeds.GenericCSVData(
dataname='./data/VXX-1d-2017-10-01us.csv',
dtformat='%Y-%m-%d',
openinterest=-1,
timeframe=bt.TimeFrame.Days,
compression=1
)
cerebro = bt.Cerebro()
cerebro.addstrategy(myStrategy)
cerebro.adddata(data)
cerebro.adddata(data_day)
cerebro.run()
cerebro.plot()
```

And get this error: `ValueError: x and y must have same first dimension, but have shapes (7542,) and (300,)`

Here is the full message:

```
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-1-6c0f97016110> in <module>
34
35 cerebro.run()
---> 36 cerebro.plot()
~\Anaconda3\envs\IBbt\lib\site-packages\backtrader\cerebro.py in plot(self, plotter, numfigs, iplot, start, end, width, height, dpi, tight, use, **kwargs)
989 rfig = plotter.plot(strat, figid=si * 100,
990 numfigs=numfigs, iplot=iplot,
--> 991 start=start, end=end, use=use)
992 # pfillers=pfillers2)
993
~\Anaconda3\envs\IBbt\lib\site-packages\backtrader\plot\plot.py in plot(self, strategy, figid, numfigs, iplot, start, end, **kwargs)
226 subinds=self.dplotsover[ind],
227 upinds=self.dplotsup[ind],
--> 228 downinds=self.dplotsdown[ind])
229
230 cursor = MultiCursor(
~\Anaconda3\envs\IBbt\lib\site-packages\backtrader\plot\plot.py in plotind(self, iref, ind, subinds, upinds, downinds, masterax)
450 xdata = np.array(xdata)[lplotmask]
451
--> 452 plottedline = pltmethod(xdata, lplotarray, **plotkwargs)
453 try:
454 plottedline = plottedline[0]
~\Anaconda3\envs\IBbt\lib\site-packages\matplotlib\__init__.py in inner(ax, *args, **kwargs)
1715 warnings.warn(msg % (label_namer, func.__name__),
1716 RuntimeWarning, stacklevel=2)
-> 1717 return func(ax, *args, **kwargs)
1718 pre_doc = inner.__doc__
1719 if pre_doc is None:
~\Anaconda3\envs\IBbt\lib\site-packages\matplotlib\axes\_axes.py in plot(self, *args, **kwargs)
1370 kwargs = cbook.normalize_kwargs(kwargs, _alias_map)
1371
-> 1372 for line in self._get_lines(*args, **kwargs):
1373 self.add_line(line)
1374 lines.append(line)
~\Anaconda3\envs\IBbt\lib\site-packages\matplotlib\axes\_base.py in _grab_next_args(self, *args, **kwargs)
402 this += args[0],
403 args = args[1:]
--> 404 for seg in self._plot_args(this, kwargs):
405 yield seg
406
~\Anaconda3\envs\IBbt\lib\site-packages\matplotlib\axes\_base.py in _plot_args(self, tup, kwargs)
382 x, y = index_of(tup[-1])
383
--> 384 x, y = self._xy_from_xy(x, y)
385
386 if self.command == 'plot':
~\Anaconda3\envs\IBbt\lib\site-packages\matplotlib\axes\_base.py in _xy_from_xy(self, x, y)
241 if x.shape[0] != y.shape[0]:
242 raise ValueError("x and y must have same first dimension, but "
--> 243 "have shapes {} and {}".format(x.shape, y.shape))
244 if x.ndim > 2 or y.ndim > 2:
245 raise ValueError("x and y can be no greater than 2-D, but have "
ValueError: x and y must have same first dimension, but have shapes (7542,) and (300,)
```

Its a resent install of BT (v1.9.67.122). I'm properly missing something simple ...Any ideas?