Combine a "plot" object with a "scatter" object in an animation

General Usage
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1

I’m creating an animation by combining multiple scatter plots (using Animation() and frame()). It all works fine. But I would like my scatter plots to be overlaid on a plot that shows the trajectories I expect the particles to follow. That plot could be done before the loop that creates the frames.

But I don’t know how to overlay a scatter on top of a plot. I don’t think scatter! is the way to go because each frame needs to be a new scatter plot. That is, I don’t want the particle positions to accumulate, but to be updated in each frame.

Here is a MWE. The particle will follow a parabolic trajectory. I want to plot that parabola underneath the particle.

using Plots
x = [-1. -1. -1.] #Starting positions
δt = 0.01
anim = Animation()
tEnd = π; t = 0;
vb = [-1,+1];

while t < tEnd
    x[3] = 2x[2] - x[1] -x[2]*δt^2 #Verlet step
    # Make plot for anim fram
    scatter([x[2]] ,[V(x[2])],xlim=vb,ylim=vb,legend=:none
      ,markersize=3,frame=:box,dpi=150,aspectratio=1,markerstrokecolor="blue",
      markercolor="blue")
    # update position vector
    x[2],x[1] = x[3],x[2]
    t += δt # advance simulation time
    frame(anim)
end
2

I don’t know how to do that using Plots, but here’s an example using Gaston and gnuplot. I couldn’t reproduce your code exactly because I don’t know what V(x) is, but my example should be easy to adapt.

using Gaston
x = -5:0.1:5;
y = sin.(π*x);
F = plot(x, y, lc = :black);  # we need one first plot
for (idx, xx) in enumerate(x)
    # plot "trajectory"
    P = plot(x, y, lc = :black, handle = 2);
    # plot "particle"
    plot!([xx], [y[idx]], w = :points,
          marker = "fcircle", markersize = 3,
          lc = :blue, handle = 2);
    push!(F, P)  # add new frame to first plot
end
# save as gif
save(term = "gif",
     saveopts = "animate size 600,400 delay 1",
     output = "anim.gif", handle = 1)

anim

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3

I can’t test this right now, but try deepcopying the trajectory plot for each frame and then use scatter! on the copy.

4

The really simple solution is just to plot the trajectory fresh in each loop. Thanks @mbaz.

using Plots
function V(x)
    return x^2
end    
tx = -1:0.02:+1
x = [-1. -1. -1.] #Starting positions
δt = 0.05
anim = Animation()
tEnd = 2π; t = 0; 
vb = [-1,+1];

 while t < tEnd
   x[3] = 2x[2] - x[1] -x[2]*δt^2
   plot(tx,V.(tx),legend=:none)
   scatter!([x[2]] ,[V(x[2])],xlim=vb,ylim=vb,legend=:none,ticks=:false
       ,markersize=3,frame=:box,dpi=150,aspectratio=1,markerstrokecolor="blue",
       markercolor="blue")
   x[2],x[1] = x[3],x[2]
   t += δt
   frame(anim)
end
gif(anim)
1 Like