Send both the time stamp and the value on the serial port.

Then Labview will know if it should plot a specific sample at 13.04 or 13.05 seconds on the time axis.

You can't just send the measurement and assume that Labview should be able to figure out the exact time (with 0.01 second resolution and precision). The PC can't even guarantee that Labview will stop receiving characters for way longer than 0.01 before it gets reactivated and then finds multiple samples cached in the input buffer.

The only time you can ignore sending time stamps is if you send an initial time stamp (or absolute time isn't needed) and Labview knows the frequency so it can step the time. But you can never let Labview depend on the PC clock to figure out an absolute time based on when the data was received.