[SalesForce] How i get a selected value from select drop down list and pass it to a method and then print (lightning)

To get the selected value of selectList in JavaScript you need to change your approach.

<apex:selectList size="1" styleclass="chartBlock">
      <apex:selectOptions value="{!dynamiclist}"></apex:selectOptions>
 </apex:selectList>

Now in your JavaScript get the value using className attribute and use that.

var e = document.getElementsByClassName("chartBlock")[0];
var strUser = e.options[e.selectedIndex].value;

you need to change javascript here

var visualization = new google.visualization.LineChart(document.getElementById('chartBlock'));

to

var e = document.getElementsByClassName("chartBlock")[0];
    var strUser = e.options[e.selectedIndex].value;
 var visualization = new google.visualization.LineChart(strUser);

There's another way to do this. You can bind an attribute in your parent component to an attribute in the modal body:

Create an attribute on the parent and on the body component.
In the controller function where you open the modal and generate the body and footer, pass a reference to the parent attribute to the body attribute. When you change the value of the modal body attribute, the value will be passed to the parent attribute through this binding.
Now you simply have to fire an onclick event to notify the parent that the Select button is clicked and to have it handle its logic. There is no need to send any parameters in the event.

$A.createComponents(
            [
                ["c:modalBody",
                    {
                        modalBodyAttributeName : component.getReference('v.parentAttributeName')
                    }],
                ['c:modalFooter', {}]
            ],
            function(contents, status, errorMessage) {
                if (status === "SUCCESS") {
                    modalBody = contents[0];
                    modalFooter = contents[1];
                    component.find('overlayLib').showCustomModal({
                        header: "header title",
                        body: modalBody,
                        footer: modalFooter,
                        showCloseButton: true,
                        closeCallback: function() {

                        }
                    });
                } else {
                    console.log(errorMessage);
                }
            }
        );