Chart.js error - bar graph not displaying

Related to the upgrade from 2.9.4 to 3.7.1 (Bug 1962118)

To test:
1. Add an institution and some people
2. Add some blocks to a page
3. Run the cron, ensuring that the weekly cron jobs run after you've created the data
4. View the reports page: Admin menu>Reports

Expected results:
 - A horizontal bar graph under the heading 'Most frequently used blocks in portfolio pages' showing number of each blocktype

Actual results:
 - No results

Errors in console:

Chart.min.js?v=8443:13 Invalid scale configuration for scale: xAxes
(anonymous) @ Chart.min.js?v=8443:13
(anonymous) @ Chart.min.js?v=8443:13
Us @ Chart.min.js?v=8443:13
(anonymous) @ Chart.min.js?v=8443:13
Js @ Chart.min.js?v=8443:13
dn @ Chart.min.js?v=8443:13
(anonymous) @ mahara.js?v=8443:1023
(anonymous) @ mahara.js?v=8443:375
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
l @ jquery.js?v=8443:2
(anonymous) @ jquery.js?v=8443:2
load (async)
send @ jquery.js?v=8443:2
ajax @ jquery.js?v=8443:2
sendjsonrequest @ mahara.js?v=8443:345
fetch_graph_data @ mahara.js?v=8443:926
(anonymous) @ statistics.php:537
e @ jquery.js?v=8443:2
t @ jquery.js?v=8443:2
setTimeout (async)
(anonymous) @ jquery.js?v=8443:2
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
fire @ jquery.js?v=8443:2
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
ready @ jquery.js?v=8443:2
B @ jquery.js?v=8443:2
mahara.js?v=8443:378 sendjsonrequest() callback failed: Error: "horizontalBar" is not a registered controller.
    at Object._get (Chart.min.js?v=8443:13:84583)
    at Object.getController (Chart.min.js?v=8443:13:83650)
    at dn.buildOrUpdateControllers (Chart.min.js?v=8443:13:94805)
    at dn.update (Chart.min.js?v=8443:13:95565)
    at new dn (Chart.min.js?v=8443:13:91945)
    at mahara.js?v=8443:1023:27
    at Object.<anonymous> (mahara.js?v=8443:375:17)
    at c (jquery.js?v=8443:2:28327)
    at Object.fireWith [as resolveWith] (jquery.js?v=8443:2:29072)
    at l (jquery.js?v=8443:2:79901) {data: {…}}
(anonymous) @ mahara.js?v=8443:378
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
l @ jquery.js?v=8443:2
(anonymous) @ jquery.js?v=8443:2
load (async)
send @ jquery.js?v=8443:2
ajax @ jquery.js?v=8443:2
sendjsonrequest @ mahara.js?v=8443:345
fetch_graph_data @ mahara.js?v=8443:926
(anonymous) @ statistics.php:537
e @ jquery.js?v=8443:2
t @ jquery.js?v=8443:2
setTimeout (async)
(anonymous) @ jquery.js?v=8443:2
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
fire @ jquery.js?v=8443:2
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
ready @ jquery.js?v=8443:2
B @ jquery.js?v=8443:2
mahara.js?v=8443:378 sendjsonrequest() callback failed: TypeError: chartobject.generateLegend is not a function
    at mahara.js?v=8443:1024:81
    at Object.<anonymous> (mahara.js?v=8443:375:17)
    at c (jquery.js?v=8443:2:28327)
    at Object.fireWith [as resolveWith] (jquery.js?v=8443:2:29072)
    at l (jquery.js?v=8443:2:79901)
    at XMLHttpRequest.<anonymous> (jquery.js?v=8443:2:82355) {data: {…}}
(anonymous) @ mahara.js?v=8443:378
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
l @ jquery.js?v=8443:2
(anonymous) @ jquery.js?v=8443:2
load (async)
send @ jquery.js?v=8443:2
ajax @ jquery.js?v=8443:2
sendjsonrequest @ mahara.js?v=8443:345
fetch_graph_data @ mahara.js?v=8443:926
(anonymous) @ statistics.php:508
e @ jquery.js?v=8443:2
t @ jquery.js?v=8443:2
setTimeout (async)
(anonymous) @ jquery.js?v=8443:2
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
fire @ jquery.js?v=8443:2
c @ jquery.js?v=8443:2
fireWith @ jquery.js?v=8443:2
ready @ jquery.js?v=8443:2
B @ jquery.js?v=8443:2
mahara.js?v=8443:378 sendjsonrequest() callback failed: TypeError: chartobject.generateLegend is not a function
    at mahara.js?v=8443:1024:81
    at Object.<anonymous> (mahara.js?v=8443:375:17)
    at c (jquery.js?v=8443:2:28327)
    at Object.fireWith [as resolveWith] (jquery.js?v=8443:2:29072)
    at l (jquery.js?v=8443:2:79901)
    at XMLHttpRequest.<anonymous> (jquery.js?v=8443:2:82355) {data: {…}}