Issue : Can only have one chartjs graph per page

[jpminnovation]

When using the chartjs engine, only the last graph is displayed on the generated HTML. This does not happen for the c3js and the chartist engines.

To reproduce the issue :

== An example

=== chart.js

chart::sample-data.csv[line,engine="chartjs"]

[WARNING]
.Nothing above
====
The graph above is not displayed, as only the last of the chart.js graph is displayed
====

=== chart.js number 2

chart::sample-data.csv[line,engine="chartjs"]

Here the graph is displayed as expected  

The result :

This issue is obtained using the following Ruby gems :

• asciidoctor 2.0.16
• asciidoctor-chart 1.0.0.alpha.1