Google Pie chart gets unformatted when called by Javascript function

Hello, guys. I am having problem rendering a graph that is with "display: none" in "display: block" via Javascript. The problem is that when I turn the chart visible it gets smaller than expected.

The graph is getting this resolution

When should I stay with this resolution

Below is the code of the graph in question

Code html

<div id="relat_sit_table" style="display:none;">
<div class='tab_title'>Situação UC Sem Medidor <?php echo "($mees/$anoo)"; ?></div><br/>
<div id='table_div_estilo2'><br/>
<div id='table_div3'></div></div><br/>
</div>
<div id="relat_sit_det2" style="display:none"><p class="relat_sit_det2"> Veja comparação entre os últimos meses <a href="#" name="relat_sit2" onClick="optionCheck4()">aqui</a></p></div>

Javascript

function optionCheck1(){
      document.getElementById("columnchart_material_2").style.display ="none";
      document.getElementById("piechart").style.display ="block";    
      document.getElementById("relat_class_det1").style.display ="none";
      document.getElementById("relat_class_det2").style.display ="block";
            }