# Gnuplot basics

## Plot an interface and a vector field

### Typing commands
~~~ literatec
gnuplot > plot [-2:2][-2:2]'velocity-100' u 1:2:($3*1e4):($4*1e4) w vec lc rgb "black", 'interface-100' u 1:2 w l lt rgb "violet"
~~~

### How to generate the file in Basilisk

*velocity-i*

~~~literatec
event velocity (i+=100) {
 
  char *outfile1 = NULL;
  outfile1 = (char *) malloc(sizeof(char) * 256);
  sprintf(outfile1, "velocity-%d", i);
  FILE * fp_velocity = fopen (outfile1, "w");
  output_field ({u.x,u.y}, fp_velocity, linear = true);
  fclose(fp_velocity);

}
~~~

*interface-i*

~~~literatec
event interface (i+=100) {
 
  char *outfile2 = NULL;
  outfile2 = (char *) malloc(sizeof(char) * 256);
  sprintf(outfile2, "interface-%d", i);
  FILE * fp_interface = fopen (outfile2, "w");
  output_facets (c, fp_interface);
  fclose(fp_interface);
}
~~~

### What's in the file
*velocity-100*

~~~literatec
# 1:x 2:y 3:u.x 4:u.y
-19.9609 -19.9609 0.00588741 -2.19209e-06
-19.9609 -19.8828 0.00588709 -6.67147e-06
-19.9609 -19.8047 0.00588784 -1.12894e-05

...
~~~

*interface-100*

~~~literatec
# 1:x_A 2:y_A
# 3:x_B 4:y_B
# \n - new line
-0.710403 0.703125
-0.703125 0.710403

-0.703125 0.710497
-0.625 0.780838

-0.780838 0.625
-0.710497 0.703125

...
~~~

for plotting AB segments

## Plot y(x)
### Typing command

~~~ literatec
gnuplot > plot 'v.dat' u 1:2 lc rgb "blue" lt 1 pt 7 ps 1.5;
~~~

### How to generate the file in Basilisk

*v.dat*

~~~literatec
event velocity(i++){
	double v_x = 0.;
	double v_y = 0.;
	double vol = 0.;
	
	foreach(){
		v_x += c[] * u.x[] * dv() ;
		v_y += c[] * u.y[] * dv() ;
		vol += c[] * dv();
	}
	
	double  V_x = v_x / vol ;
	double  V_y = v_y / vol ;
	
  FILE * fp_v = fopen ("v.dat", "a");
  fprintf(fp_v,"%d %g %g\n",i,V_x,V_y);
  fclose(fp_v);
}
~~~

### What's in the file

*v.dat*

~~~ literatec
# 1:i 2:u.x 3:u.y
0 0 0
1 -0.18086 0.180693
2 -0.000273179 0.000202897
3 0.00270616 -0.00317989

~~~

## Study the evolution of a tracer for a given abscisse or ordinate

The function [interpolate()](http://basilisk.fr/src/grid/cartesian-common.h#598) allows to take the values for given coordinates.

~~~ literatec
event print (i+=10) {
 
 for (double y = -L/2 ; y <= L/2; y += 0.01){
 
 char *outfile3 = NULL;
 outfile3 = (char *) malloc(sizeof(char) * 256);
 sprintf(outfile3, "D-%d", i);
 FILE * fp_D = fopen (outfile3, "a");
 fprintf(fp_D,"%g %g \n", y, interpolate (c, 0, y));
 fclose(fp_D);	
 }
}
~~~

# Gnuplot video

## Typing commands

~~~literatec
gnuplot > n=0
gnuplot > load 'file_name.gp'
~~~

## 'file_name.gp'

### Interface and vector field
~~~literatec
interface(n) = sprintf("interface-%d", n)
velocity(n)  = sprintf("velocity-%d", n)

if ( n <= 1000 )

plot [-3:3][-2:2] interface(n) u 1:2 w l lt rgb "violet", velocity(n) u 1:2:($3*1):($4*1) w vec lc rgb "black"

n=n+1
pause 0.1 // if there is not a lot of points, the plots appear too quick, so a pause time  [s] is needed
reread
~~~

### XYZ splot - gradient of sigma in x direction

~~~literatec
Gx_sigma(n) = sprintf("Gx_sigma-%d", n)

if ( n <= 10 )

splot Gx_sigma(n) u 1:2:4 w lines
// ou splot Gx_sigma(n) u 1:2:4 w pm3d

pause 1
n=n+1
reread
~~~

## How to generate the file in Basilisk

###[Velocity and interface file generation](#how-to-generate-the-file-in-basilisk)

### Gradient of sigma in x direction

~~~literatec
   for (scalar c in interfaces){
   
     scalar sigma_temp = c.sigma;

       foreach(){ 

          double Gx_sigma = sigma_temp[] - sigma_temp[-1,0] ; 
    
	  char *outfile3 = NULL;
          outfile3 = (char *) malloc(sizeof(char) * 256);
          sprintf(outfile3, "Gx_sigma-%d", i);
          FILE * fp_Gx_sigma = fopen (outfile3, "a");
          fprintf(fp_Gx_sigma,"%g %g %g %g %g\n",x,y,Gx_sigma, sigma_temp[], sigma_temp[-1,0]);
          fclose(fp_Gx_sigma);

       }
   }
~~~