1. A computer-implemented method comprising: identifying, by one or more computing devices, a portion of a map for presentation on a device; identifying, by the one or more computing devices, a set of content items in which each content item is associated with a geographic location that is presented in the portion of the map; determining, by the one or more computing devices, for a given content item from the set of content items, an adjusted value of the given content item based at least in part on a base value for the content given item and a set of adjustment factors for one or more other content items in the set of content items; selecting, by the one or more computing devices, one or more of the content items from the set of content items based on the adjusted values of the one or more content items; and providing, by the one or more computing devices, over a network and to the device, data that present the selected content items on the portion of the map presented at the device.