A single indicator instance can only plot to a single panel. That panel can either be the chart panel or a separate panel. You can control this in your indicator code by using IsOverlay.
https://ninjatrader.com/support/helpGuides/nt8/isoverlay.htm
You can’t send different plots from the same indicator instance to different panels. That is not supported as far as I know.
In your case, if all you’re trying to plot is the one signal for buy/sell/nothing, then you can define a single plot and just assign it values of 1, 0, -1 as you suggested. You would only use Values[0][0] for assignments. Values[0] refers to the first plot (index 0), Values[1] refers to the second plot (index 1), so and so forth. Since you only have one plot, you will only use Values[0].
In OnBarUpdate:
if (my calculation is bullish)
{
Values[0][0] = 1;
}
else if (my calculation is bearish )
{
Values[0][0] = -1;
}
else
{
Values[0][0] = 0;
}
Hope this helps.