See this post, please: https://goo.gl/ltXjBC

_30_ATTINY_pwmGraphicGeneration.ino_

#include <FlexiTimer2.h>                // use timer for sampling to get even time base for data

const int READ_A0_PIN = A0;
const int READ_A1_PIN = A1;
const int READ_A2_PIN = A2;



int inverted_value = 0;

void setup() {
  Serial.begin(115200);                 // initialize serial communications (to match that used by PlotNValues.pde)
  pinMode(READ_A0_PIN, INPUT);
  pinMode(READ_A1_PIN, INPUT);
  pinMode(READ_A2_PIN, INPUT);

  FlexiTimer2::set(10, sample);
  FlexiTimer2::start();
}

void loop() {
  //wait interruptions
}

// sample values and pipe to processing
void sample() {
  String result = "";
  Serial.println(result + analogRead(READ_A0_PIN) + "," + analogRead(READ_A1_PIN) + "," + analogRead(READ_A2_PIN));
}