You can accomplish this using the Any Chart widget, which sits on top of ChartsJS. This chart widget does have a version compatible with Mx Version 7.18 if you look under the ‘older versions’ tab on the app store page.
How can you make this a combined chart? The beauty and frustration of the Any Chart widget is that you have to build the JSON for your chart yourself. You can use variables, but instead of just passing Mendix values you are going to need to programmatically generate the ‘data’ JSON that Charts JS requires. It will take a bit of work, bit it is doable.
Charts JS provies you with a way to specifiy your Data JSON to have two types of chart and an example of that can be found on the documentation listed below: