The ornamental design for a datalogger, as shown and described.
FIG. 1 is a front view of the data logger showing my our design according to a first embodiment, wherein the front view of the data logger is the same as the rear view thereof;
FIG. 2 is a left view thereof which is the same as the right view according to the first embodiment;
FIG. 3 is a top view thereof according to the first embodiment;
FIG. 4 is a bottom view thereof according to the first embodiment; and,
FIG. 5 is a respective view thereof according to the first embodiment.