This ProBuilder code snippet demonstrates how to plot the last specified number of levels of the Donchian Channel, using a 500-period lookback. The lines are extended across the chart to visually represent the upper and lower bounds of the channel dynamically as they change.

defparam drawonlastbaronly=true
maxlines = 20 //how many lines to plot?
hh= DonchianChannelUp[500]
ll= DonchianChannelDown[500]

if hh<>hh[1] then
    $hh[hhi]=hh
    hhi=hhi+1
endif

if ll<>ll[1] then
    $ll[lli]=ll
    lli=lli+1
endif

if islastbarupdate then
    count=0
    for i = max(hhi,lli) downto 0 do
        if isset($hh[i]) then
            drawhline($hh[i]) coloured(0,200,0)
        endif
        if isset($ll[i]) then
            drawhline($ll[i]) coloured(200,0,0)
        endif
        count=count+1
        if count>=maxlines then
            break
        endif
    next
endif

return

The code snippet above is structured to dynamically track and display the upper and lower bounds of the Donchian Channel over the last 500 periods, but only plots the most recent changes up to a specified number of lines. Here’s a step-by-step breakdown:

• Initialization: The defparam drawonlastbaronly=true ensures that drawing commands are executed only on the last bar update to optimize performance.
• Configuration: maxlines sets the maximum number of lines to display, limiting the number of historical channel levels shown.
• Channel Calculation: hh and ll store the current upper and lower values of the Donchian Channel with a period of 500.
• Storing Changes: Changes in the channel’s upper and lower bounds are tracked. If a change occurs, it is stored in arrays $hh and $ll, indexed by hhi and lli respectively, which are incremented upon each change.
• Plotting Lines: If it’s the last bar update, the code iterates from the most recent change backwards, plotting horizontal lines for each stored upper and lower bound until the maximum number of lines (maxlines) is reached.
• Color Coding: Upper bounds are colored green (coloured(0,200,0)) and lower bounds are colored red (coloured(200,0,0)), providing a clear visual distinction.

This approach is useful for traders or analysts who want to visualize significant levels of support and resistance over a given period, helping to identify potential breakout or reversal points.