Constant crashing of TEMPERATURE ON A PCD8544 DISPLAY, WITH DS18B20 TEMPERATURE SENSOR

My next slight mystery (apart from not surviving a reset/power cycle) is that the font has become very ugly. I am not sure at what point this happened, and actually thought it might be the LCD, so replaced. But same effect ...

SPI1.setup({ baud: 1000000, sck:B3, mosi:B5 });
var ow = new OneWire(B13);
var g, temp;
function onInit() {
  clearInterval();
  temp = require("DS18B20").connect(ow);
  g = require("PCD8544").connect(SPI1,B6,B7,B8, function() {
    setInterval(onTimer, 500);
  });
}
function onTimer() {
  // Get the temperature
  var t = temp.getTemp();
  // Round it to the nearest 0.1
  t = Math.round(t*10)/10;
  // Now draw!
  g.clear();
  g.setFontBitmap(); // simple 8x8 font
  g.drawString("Temp",2,0);
  g.drawLine(0,10,84,10);
  g.setFontVector(25); // large font
  g.drawString(t, 0, 15);
  g.flip(); // copy this to the screen
}
onInit();

Incidentally the default 0,0 for Temp, puts the text off the LHS of the screen slightly, so I changed to 2,0. Changing setFontVector to other numbers has little effect .

Is this to be expected/as intended ?

Pat