Every function has its own workspace. Your function does not know x_data, y_data, accel_data and velocity_data. There are several ways to pass variables between callbacks and for more details about this topic please read this article ( link).
It seems that your are working with GUIDE, so you could append your variables to your handles structure. I am missing the part of your code were your variables origin from, it could look like this:
function MyDataCreation(hObject,event,handles)
handles.x_data=1:3;
handles.y_data=4:6;
handles.accel_data=7:9;
handles.velocity_data=10:12;
guidata(hObject,handles)
end
In your export function:
function export_data_callback(hObj, event, handles)
handles=guidata(hObj);
%now you can use handles.x_data, handles.y_data ...
%rest of your code
%
end
You can either use uiputfile or inputdlg to ask for a filename.
As soon as you have your variables in your function there is no need to use evalin anymore.
