I have figured out one way of solving it.
Since the MQTT Publish block does not support Strings, it's possible to send raw data to Tago.io platform and use the Payload Parser to format the data being received.
In Simulink, one should add the String to ASCII block before the Publish.
On Tago.io parser, one should write a small script in JS getting the data from a raw payload and comnverting to desired data type. Then it will be trivial to find the new variable for plotting.