This gives basic information : Arduino Playground - MQGasSensors
What you want is not possible.
The resistor is the only way to adjust the sensor output.
You could use the resistor as "Adjust".
And you could use software for "Sensitivity", using the map() function : map() - Arduino Reference
If the bar on the LCD is the only output, it is very rough. The analog value could easily be mapped for the LCD bar with the map() function.