1. A method, comprising the steps of: inserting a probe into a heart of a living subject, the probe having a plurality of electrodes; recording electrograms from the electrodes at respective locations in the heart; determining slopes and annotations in the electrograms within time windows; establishing relationships among the slopes and annotations from different ones of the electrograms; and determining from the relationships lines of conduction block in the heart; wherein determining slopes and annotations comprises the steps of: determining bipolar windows in the electrograms; annotating local activation times within the bipolar windows; determining from readings of a set of electrodes that a block point exists in a region of the set of electrodes; repositioning the local activation times responsively to the block point; and determining revised windows that include respective local activation times; and wherein establishing relationships among the slopes and annotations comprises: identifying primary slopes and secondary slopes in the electrograms; determining whether the electrodes are in contact with the heart; and determining whether the primary slopes and the secondary slopes are coupled to one another.