1. An apparatus, comprising: electrical circuitry for recording signal data from electrodes of a probe when the probe is at a location in a heart of a living subject, the signal data comprising a bipolar electrogram and a unipolar electrogram; a memory for storing the signal data; a display; and a processor connected to the memory and operative for performing the steps of: defining a time interval comprising a window-of-interest; differentiating the bipolar electrogram and the unipolar electrogram with respect to time to define a differentiated bipolar electrogram and a differentiated unipolar electrogram, respectively; identifying peaks within the window-of-interest in the differentiated bipolar electrogram; establishing a bipolar activity window about respective peaks, the bipolar activity window having activity bounds and comprising a time interval of activity about the peaks in the bipolar electrogram; and identifying an extreme negative value (−dV/dt) in the differentiated unipolar electrogram within the activity bounds, and reporting a time corresponding to the value as a unipolar activation onset on the display; wherein identifying an extreme negative value comprises: defining a slope window in the unipolar electrogram that contains downward sloping intervals; fitting respective regression lines to the downward sloping intervals; determining a trend in the slope window, the trend having a slope; identifying a longest monotonic downward sloping interval contained in each slope window; making a determination that the longest monotonic downward sloping interval occupies less than 50% of its containing slope window; and responsive to the determination, subtracting the slope of the trend from a slope of the regression line of the longest monotonic downward sloping interval.