Creating formatted String (including floats) in Arduino-compatible C++
The Adafruit_GFX class has two methods named print() and
println(). These behave exactly like Serial.print() and
Serial.println(). Specifically, when printing a float, you can provide
a second argument telling it the number of decimal places you want after
the decimal point:

display.print(temp1, 1);  // 1 digit after the decimal point
display.print(", ");
display.print(hum1, 1);  // ditto
display.print("%");


Background: the reason these methods behave like the ones of
Serial is that both Adafruit_GFX and HardwareSerial inherit them
from the Print virtual class.