In its ten episode season, Half and Catch Fire averaged 800,000 viewers and 0.3 A18-49 demo. While that may not look horrible, the show debuted to 1.19 million viewers and 0.3 demo, before collapsing to 574,000 viewers and 0.2 demo for its season finale. That is the lowest performance for an episode of an AMC original drama in the networks history (since Mad Men premiered in 2007). Yes, lower than Low Winter Sun and Rubicon, both of which last one season.
AMC says the renewal is based on a committed core audience and critical acclaim. In addition, Halt and Catch Fire is one of the most upscale dramas on television in the A25-54 demographic (behind Mad Men, The Good Wife, and Parenthood, respectively).