Your first step should be to write a small sketch that subscribes to the topic on the MQTT broker and when the hydrometer publishes data to it, read the data and print it
Forget all about the graphing the data until that works. Have you looked at the examples from the PubSubClient library ? Which Arduino board are you using ?
Is the MQTT broker a local one or is it on the Internet and if so what is its address and the name of the topic to which the data will be published ?