The plugin is https://www.ads-software.com/plugins/front-end-editor/
The sparklines work if Front End Editor is disabled.
The Javascript console prints the following errors:
[21:25:28.406] TypeError: $jSpark(...).sparkline is not a function @ https://example.com/:697
[21:25:28.425] Error: Mismatched anonymous define() module: function (a){"use strict";var b={},c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I=0;c=function(){return{common:{(...)},line:{(...)}">●</s @ https://example.com/wp-content/plugins/front-end-editor/lib/aloha-editor/lib/aloha.js:80