How to set a Static threshold value on Line chart

Here is my search for transaction response times on web logs: index=bridger sourcetype=bridger_wbs_txns User_ID=rtm_primary_1 | eval "Transaction time"= Trans_time | timechart max(Transaction time), min(Transaction time), avg(Transaction time) I want to display a static line across the chart which represents 4.5seconds as a threshold value. That way, I can see which transactions are exceeding threshold. At times, threshold values change with SLAs. How can this be accomplished? thanks, Anne