[amibroker] How do you plot the "bid" value of a foreign symbol?

[ymtrader1]

I thought the following code would draw the changing value of the
"bid" value but instead it just draws a horizontal line of the current
value:

upvol=GetRTDataForeign("BID","vol-nasd-nasdaq-ind");
Plot(upvol,"UVol",colorRed,stylecandle);

It appears that upvol is being treated as a constant value instead of
an array of values.  How do I define upvol as an array so I can plot
the "upvol" data into a graph of the changing values?


Thanks,