title: plotting equations with restricted domains in shiny
body: <p>there. 
  Im building an shiny app thats plots some functions. The user can modify the parameters.  The problem appears when I have a restricted function,  specially when its related to x. This is one example:</p>
  
  <p>Sliders:</p>
  
  <pre><code>sliderInput("th19",
      HTML("$$ \\theta_1 $$"),
      min = 1,
      max = 10,
      value = 2)
  sliderInput("thB9",
      HTML("$$ \\theta_b $$"),
      min = 1,
      max = 10,
      value = 2)
  sliderInput("vthB9",
     HTML("$$ \\vartheta_b $$"),
     min = 1,
     max = 20,
     value = 2)
  </code></pre>
  
  <p>Plot:</p>
  
  <pre><code>mForm9.1 &lt;- as.formula("Y ~ vthB9 + th19*(x - thB9)")
  mExpr9.1 &lt;- mForm9.1[[3]]
  
  output$Curve9 &lt;- renderPlot({
      th19 &lt;- input$th19
      vthB9 &lt;- input$vthB9
      thB9 &lt;- input$thB9
  
      eval(call("curve", mExpr9.1, col = 2, ylab = "", main =
                 expression(vartheta[b] + theta[1]*(x - theta[b]))))
  
      },  height = 400, width = 600)
  
      mainPanel(
          tabsetPanel(type = "tabs", 
              tabPanel("Gráfico", plotOutput("Curve9"))
     ))
  </code></pre>
  
  <p>What happens here is that when 'x' is bigger than 'vthB9', the equation resumes itself to only 'vthB9', and this is only one of the cases I have. Anybody knows what to do?</p>
  
  <p>*Hope I have been clear</p>
  
  <p>*Im using <code>flexdashboard</code>, this is why the shiny might seen a little different </p>