I don't know why it is. A spark-like graph is useful for many use-cases (graph or bars? plot this, draw lines), and even more a thermometer-like icon (weather, medicine.) The thermometer has been hanging around the issues for a while also (or so I think from last time I checked.) I'd like to have all my icons in one basket, and I like the look of Font Awesome's. So missing these 2 is a little aesthetic pain.