1. A method for automatically monitoring a patient by an implantable medical device, comprising: receiving signals from each real axis of a multi-axis sensor by a processor of the implantable medical device; generating from the real axis signals a virtual signal for each of a plurality of virtual axes of the multi-axis sensor; determining a first value of an activity metric during a first activity state of the patient for each of the real axis signals and each of the virtual signals; determining a second value of the activity metric during a second activity state of the patient for each of the real axis signals and each of the virtual signals; identifying by the processor an optimal signal axis from the real axes and the plurality of virtual axes for monitoring a physiological signal of the patient by identifying one of the real axis signals and the virtual signals having at least one of a lowest variation of the first value of the activity metric during the first activity state and a greatest difference between the first value and the second value of the activity metric; storing a set of coordinates defining the optimal axis as respective weighting factors of the signals received from each real axis of the multi-axis sensor; monitoring the physiological signal of the patient by determining a third value of the activity metric using the multi-axis sensor signals and the stored weighting factors; and controlling a rate of delivering a therapy to the patient by a signal generator of the implantable medical device based on the third value of the activity metric.