I am using Splunk's "Website Monitoring" app to check the status of a few URLs. The output gives code 200 if the URLs are accessible. There are three columns in the table - url, responsecode and responsetime (in milliseconds).
You can configure an overlay comfortably via the gui as well. Click the "Format" button (the one with the brush next to the one where you select the type of chart) and go to "Chart Overlay". From there, you can add the series you want to display as overlay and configure whether to use the x axis on the right hand of the chart (which you want) by selecting "View as Axis".