The ornamental design for a datalogger, as shown and described.
FIG. 1 is a front top perspective view of a data logger showing our new design;
FIG. 2 is a rear top perspective view thereof;
FIG. 3 is a front bottom perspective view thereof;
FIG. 4 is a rear bottom perspective view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a left elevational view thereof;
FIG. 8 is a right elevational view thereof;
FIG. 9 is a front elevational view thereof; and,
FIG. 10 is a rear elevational view thereof.
The broken lines shown in the drawings depicting protrusions, recesses and other features illustrate environment that forms no part of the claimed design.