String y = "2*y+1";
int x = y;
drawgraph(x);

import javax.script.*;

interface GraphFunction {
    double eval(double x);

    public static GraphFunction createFromString(String expression) {
        try {
            ScriptEngine engine = new ScriptEngineManager()
                .getEngineByName("JavaScript");
            engine.eval("function graphFunc(x) { return " + expression + "; }");
            final Invocable inv = (Invocable)engine;
            return new GraphFunction() {
                @Override
                public double eval(double x) {
                    try {
                        return (double)inv.invokeFunction("graphFunc", x);
                    } catch (NoSuchMethodException | ScriptException e) {
                        throw new RuntimeException(e);
                    }
                }
            };
        } catch (ScriptException e) {
            throw new RuntimeException(e);
        }
    }
}

class Test {
    public static void main(String[] args) {
        GraphFunction f = GraphFunction.createFromString("2*x+1");

        for (int x = -5; x <= +5; x++) {
            double y = f.eval(x);
            System.out.println(x + " => " + y);
        }
    }
}

-5 => -9.0
-4 => -7.0
-3 => -5.0
-2 => -3.0
-1 => -1.0
0 => 1.0
1 => 3.0
2 => 5.0
3 => 7.0
4 => 9.0
5 => 11.0