You need to use project before the render to select just the columns you want to appear in the chart

| project timestamp, duration

so in full would be:

availabilityResults
| where timestamp > ago(24h) //set the time range
| where name == "sitename" //set the monitor name 
| project timestamp, duration
| render areachart with (xcolumn=duration,ycolumns=timestamp)

https://learn.microsoft.com/en-us/kusto/query/tutorials/learn-common-operators?view=microsoft-fabric#select-a-subset-of-columns