How can I set the data type to enable charts?

I included some values from vzlogger which are numeric. Nevertheless, clicking on the value just shows the “coloured bar” instead of nice curvy charts. What am I missing?

Example config

platform: rest
  resource: "http://host:8080"
  value_template: > 
    {% for i in %}
        {% if i.uuid == "32754b70-f5d9-11e5-afa4-xxxxxx" %}
        {{ i.tuples[0][1] }}
        {% endif %}
    {% endfor %}

  method: GET
  name: Power L1
  unit_of:measurement: "Ampere"

You seem to have a typo there.
unit_of:measurement: “Ampere”
should be
unit_of_measurement: “Ampere”