How to plot only during part of the session

You cannot move the plot call inside a branch: every chart-output function must be called at the top level. Vary the value instead. An na in a plotted series leaves a gap on that bar rather than drawing a zero, so plot na outside the window.

indicator("Close during the morning", "am", true)

bool in_window = hour >= 9 and hour < 12

plot(in_window ? close : na, title = "Morning close", color = color.blue)

hour and minute are series int values read off the bar under evaluation. Build the test from those.

If the window needs the instrument’s timezone

The bare hour variable reads the current bar; this edition does not state which timezone it is expressed in. Where that matters, call the function of the same name, which takes a time and a timezone and requires both.

hour(series int time, series string timezone) → series int

hour(time, syminfo.timezone)

The instrument’s own timezone is syminfo.timezone, a simple string.

If you would rather mark the window than hide the plot

Keep the plot continuous and put the condition into a marker. plotshape takes the condition as its first argument, so the marker appears on the bars in the window and nowhere else.

indicator("Morning bars", "ammark", true)
plot(close, title = "Close", color = color.gray)
plotshape(hour >= 9 and hour < 12, title = "Morning", shape = shape.circle, location = location.belowbar, color = color.blue, size = size.tiny)

The failure to watch for

Writing the window test as a branch around the plot call is rejected with an error saying the call must be made in the global scope. The condition belongs in an argument. See Plotting.