<div class="container collapse">
    <div class="row form-group">


        <div class="col-md-6">
            <div class="col-md-12" style="height:300px;">
                <canvas id="graph1"></canvas>
            </div>
        </div>
        <div class="col-md-6">
            <div class="col-md-12" style="height:300px;">
                <canvas id="graph2"></canvas>
            </div>
        </div>

    </div>

    <div class="row form-group">


        <div class="col-md-6">
            <div class="col-md-12" style="height:300px;">
                <canvas id="graph3"></canvas>
            </div>
        </div>
        <div class="col-md-6">
            <div class="col-md-12" style="height:300px;">
                <canvas id="graph4"></canvas>
            </div>
        </div>

    </div>
</div>