title: Observed curve from az.plot_ppc does not match kde plot of actual data
body: <p>I am modeling data that I think is gamma distributed so I write a PyMC3 model as below.</p>
  <pre><code>import pymc3 as pm
  import arviz as az
  
  with pm.Model() as model2:
      alpha = pm.Exponential('alpha', 5)
      beta = pm.Exponential('beta', 5)
      
      p_factor = pm.Gamma('p_factor', alpha = alpha, beta = beta, observed = d.productivity_factor)
      
      prior_checks = pm.sample_prior_predictive(samples=100, random_seed= seed)
      
      trace2 = pm.sample(2000, tune=2000)
  
  </code></pre>
  <p>I then plot a posterior predictive check</p>
  <pre><code>with model2:
      ppc2 = pm.sample_posterior_predictive(trace2, var_names=[&quot;alpha&quot;, &quot;beta&quot;,'p_factor'], random_seed=seed)
      
  idata = az.from_pymc3(trace2, posterior_predictive=ppc2)
  az.plot_ppc(idata)
  
  </code></pre>
  <p>but the observed curve does not match what I expect. Is there something I am doing wrong?</p>
  <p><img src="https://i.stack.imgur.com/x9HWr.png" alt="KDE plot of observed data" /></p>
  <p><img src="https://i.stack.imgur.com/VZs0U.png" alt="PPC Plot" /></p>