Error when zooming chart on chart.Posn

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Ok finally found time to create reproducible example on this error which I seem to encounter randomly. On most periods zoom_Chart works but sometimes I get this error:

> Error in plot.window(c(1, 61), c(NaN, NaN)) : need finite 'ylim' values
> In addition: Warning messages:
> 1: In min(x) : no non-missing arguments to min; returning Inf
> 2: In max(x) : no non-missing arguments to max; returning -Inf

And here is my reproducible example:

```
library(blotter);

currency("USD");

stock("SPY",currency="USD",multiplier=1);

initDate <- '2006-01-01';
startDate <- '2006-01-01';
endDate <- '2016-03-16';
initEq <- 10000;

Sys.setenv(TZ = "UTC");

getSymbols('SPY', from=startDate, to=endDate, index.class="POSIXct", adjust=T)

addTxn("TEST", Symbol='SPY', TxnDate=startDate, TxnPrice=SPY[1,]$SPY.Close, TxnQty = 100, TxnFees=0, verbose = TRUE)

chart.Posn("TEST", Symbol = 'SPY')
zoom_Chart("2008-01::2008-03")
```

Run the supplied R example with chart.Posn("TEST", Symbol = 'SPY') and zoom_Chart("2008-01::2008-03") to reproduce the non-finite ylim error. Trace those entry points to determine why the selected period has no finite values, then verify that zooming this period completes without the error or warnings.

Written by the indexing model from the issue text.