Bug fix for single data point handling in iocost_tune benchmark

iocost_tune aborts with the following when a data series has one data point:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: invalid input order (DataPoint { x: 122.03, y: 0.94 }, DataPoint { x: 122.03, y: 0.94 })', /home/demo/.cargo/registry/src/github.com-1ecc6299db9ec823/resctl-bench-2.2.2/src/bench/iocost_tune.rs:1633:18

Fix it by fitting a flat line (or dot) on the single data point.