Most likely the path in the cronjob is incorrect. Make sure that the file really exists in the path you provide. Copy and paste the whole line (remove * symbols etc) and see what happens.
Start editing a page that will display the graph. The icon you need to click is here:   black_sun
First make sure that everything is wired correctly. Take a look at the wiring pictures and double check. Afterwards run the script manually and check if any readings are taken. Double check the plugin shortcode. Check if the readings are stored in the database. Do that by typing mysql -u root -p and then type in your mysql password. Type in show databases;which will display all databases, select the WordPress database. In my case it is named WordPressDB, I can select it with use WordPressDB;.  The table that stores all the readings is named temperatures. Display everything in it with select * from temperatures;. It should display all the readings taken. If all of the above works, it is very likely that you have a faulty sensor.

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