I think to show those values on the Y-Axis you would have convert them to numbers. If you're on Elasticsearch and Kibana 5.0 you can use scripted fields in Kibana with the Painless language. I made one scripted field with this script;
if (doc['name.keyword'].value == "a") return 1;
if (doc['name.keyword'].value == "b") return 2;
if (doc['name.keyword'].value == "c") return 3;
if (doc['name.keyword'].value == "d") return 4;
return 0;
Note that the fields you use like my name.keyword above can not be analyzed strings. If they are strings that have to be keywords.
Or maybe you can use filters, but I don't think you can get the chart to show the string values on the y-axis. In the chart below you see I've added filters for a field geo.dest equaling a certain value. And then I'm getting the count.
You can also concatenate or doing other string manipulation with Painless scripted fields.
Let me know if you need help with that.
Regards,
Lee