This will plot the intraday high and low of the current day on a 1 minute chart. Just change 0830 to whatever time zone you are in for the open. If you use a longer interval than 1 minute, divide MinSinceOpen by the appropriate interval in the plot statement (Ex. Plot1(Highest(High, MinSinceOpen/5), "High"); for a 5 minute chart, etc.)
{Start code}
Vars: MinSinceOpen(0), StartMin(0);
If Time = 0830 then StartMin = TimeToMinutes(0830);
MinSinceOpen = TimeToMinutes(Time) - StartMin;
Plot1(Highest(High, MinSinceOpen), "High");
Plot2(Lowest(Low, MinSinceOpen), "Low");
{End code}
Make sure you plot the indicator as points rather than a line so that each new high or low is not connected together.