<style name="echelle_relecture">
    <item name="android:layout_width">fill_parent</item>
    <item name="android:layout_height">wrap_content</item>
    <item name="android:background">@drawable/btn_relecture_echelle</item>
    <item name="android:button">@null</item>
    <item name="android:layout_weight">1</item>
    <item name="android:layout_marginLeft">2dp</item>
    <item name="android:layout_marginRight">2dp</item>
    <item name="android:layout_gravity">center_horizontal</item>
</style>

<RadioGroup android:layout_height="wrap_content" android:id="@+id/radioGroup1" android:layout_width="fill_parent" android:orientation="horizontal" android:layout_marginTop="10dp">
        <RadioButton style="@style/echelle_relecture" android:text="@string/day" android:id="@+id/jour" android:checked="true"></RadioButton>
        <RadioButton style="@style/echelle_relecture" android:text="@string/hour" android:id="@+id/hour" android:button="@null"></RadioButton>
        <RadioButton style="@style/echelle_relecture" android:text="@string/min" android:id="@+id/mins" android:button="@null" android:enabled="false"></RadioButton>
</RadioGroup>

android:gravity="center"