The Visual Micro plugin for Microsoft Visual Studio 2015 Community Edition (free) provides a USB debugger for Arduino. It allows you to do exactly as you describe. (Don't forget to click to install C++ during IDE install)
The debugger supports serial, RF, Bluetooth and some Wi-Fi. It enables the values of variables to be watched or update while the Arduino runs. Chart and data/pin visualizations along with watch, trace, break are included.
News: January 2016 also includes release of a GDB debugger for the Arduino Zero. As with the Serial/Bluetooth debugger, the GDB version supports ino/cpp source code, but additionally supports debug of Arduino core and libraries. It supports many features such as: step over, step out, step instruction or source line, memory, registers, locals, watch, live expressions, and stack trace.
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