charts not displaying

I have used just the basic shortcode [gfchartsreports]. My form ID=1 has over 40 responses and all questions have answers. However, when I view the page I can tell that there are placeholder squares for charts, but the charts are invisible. And there are also a number of messages stating that certain questions have no responses, which is untrue.

I am running the following:
PHP Version 7.1.2
MySQL Version 5.6.33
WordPress Version 4.7.3
Gravity Forms Version 2.1.3
Add-Ons: Mailchimp, Surveys, Partial Entries